diff --git a/specs/base/contentTypes/LazyContentDataSpec.ts b/specs/base/contentTypes/LazyContentDataSpec.ts
index b7854106..47ee8289 100644
--- a/specs/base/contentTypes/LazyContentDataSpec.ts
+++ b/specs/base/contentTypes/LazyContentDataSpec.ts
@@ -8,14 +8,14 @@ function createTestResourceResolver(): ResourceResolver {
     resolveUri(uri: string) {
       return uri;
     },
-    async resolveData(uri: string): Promise<Buffer | null> {
-      return null;
+    async resolveData(uri: string): Promise<Buffer | undefined> {
+      return undefined;
     },
     async resolveDataPartial(
       uri: string,
       maxBytes: number
-    ): Promise<Buffer | null> {
-      return null;
+    ): Promise<Buffer | undefined> {
+      return undefined;
     },
     derive(uri: string): ResourceResolver {
       return this;
diff --git a/src/base/contentTypes/BufferedContentData.ts b/src/base/contentTypes/BufferedContentData.ts
index 7f9c1ed2..1386df27 100644
--- a/src/base/contentTypes/BufferedContentData.ts
+++ b/src/base/contentTypes/BufferedContentData.ts
@@ -32,7 +32,7 @@ export class BufferedContentData implements ContentData {
    * @returns The ContentData
    */
   static create(uri: string): ContentData {
-    let data: Buffer | null = null;
+    let data: Buffer | undefined = undefined;
     try {
       data = fs.readFileSync(uri);
     } catch (error) {
@@ -61,10 +61,10 @@ export class BufferedContentData implements ContentData {
   private readonly _magic: Buffer;
 
   /**
-   * The content data, or `null` if the data could not
+   * The content data, or `undefined` if the data could not
    * be resolved.
    */
-  private readonly _data: Buffer | null;
+  private readonly _data: Buffer | undefined;
 
   /**
    * The object that was parsed from the content, assuming
@@ -86,7 +86,7 @@ export class BufferedContentData implements ContentData {
    * @param uri - The URI of the content data
    * @param data - The actual content data buffer
    */
-  constructor(uri: string, data: Buffer | null) {
+  constructor(uri: string, data: Buffer | undefined) {
     this._uri = uri;
     this._extension = path.extname(uri).toLowerCase();
     if (data) {
@@ -112,7 +112,7 @@ export class BufferedContentData implements ContentData {
 
   /** {@inheritDoc ContentData.exists} */
   async exists(): Promise<boolean> {
-    return this._data !== null;
+    return this._data !== undefined;
   }
 
   /** {@inheritDoc ContentData.magic} */
@@ -121,7 +121,7 @@ export class BufferedContentData implements ContentData {
   }
 
   /** {@inheritDoc ContentData.data} */
-  async getData(): Promise<Buffer | null> {
+  async getData(): Promise<Buffer | undefined> {
     return this._data;
   }
 
diff --git a/src/base/contentTypes/ContentData.ts b/src/base/contentTypes/ContentData.ts
index d4afbc7b..215d2f56 100644
--- a/src/base/contentTypes/ContentData.ts
+++ b/src/base/contentTypes/ContentData.ts
@@ -36,10 +36,10 @@ export interface ContentData {
   /**
    * Returns the actual content data that was read from the URI.
    *
-   * @returns The promise to the data, or to `null` when the
+   * @returns The promise to the data, or to `undefined` when the
    * data could not be obtained.
    */
-  getData(): Promise<Buffer | null>;
+  getData(): Promise<Buffer | undefined>;
 
   /**
    * Returns the object that was parsed from the content data,
diff --git a/src/base/contentTypes/LazyContentData.ts b/src/base/contentTypes/LazyContentData.ts
index ea89e697..4c913c22 100644
--- a/src/base/contentTypes/LazyContentData.ts
+++ b/src/base/contentTypes/LazyContentData.ts
@@ -50,10 +50,10 @@ export class LazyContentData implements ContentData {
   private _magic: Buffer | undefined;
 
   /**
-   * The content data, or `null` if the data could not
+   * The content data, or `undefined` if the data could not
    * be resolved.
    */
-  private _data: Buffer | null;
+  private _data: Buffer | undefined;
 
   /**
    * Whether the `_data` was already requested
@@ -87,7 +87,7 @@ export class LazyContentData implements ContentData {
     this._resourceResolver = resourceResolver;
     this._extension = path.extname(uri).toLowerCase();
     this._magic = undefined;
-    this._data = null;
+    this._data = undefined;
     this._dataWasRequested = false;
     this._parsedObject = undefined;
     this._parsedObjectWasRequested = false;
@@ -112,7 +112,7 @@ export class LazyContentData implements ContentData {
       this._uri,
       1
     );
-    this._exists = partialData !== null;
+    this._exists = partialData !== undefined;
     return this._exists;
   }
 
@@ -137,13 +137,13 @@ export class LazyContentData implements ContentData {
   }
 
   /** {@inheritDoc ContentData.getData} */
-  async getData(): Promise<Buffer | null> {
+  async getData(): Promise<Buffer | undefined> {
     if (this._dataWasRequested) {
       return this._data;
     }
     this._data = await this._resourceResolver.resolveData(this._uri);
     this._dataWasRequested = true;
-    this._exists = this._data !== null;
+    this._exists = this._data !== undefined;
     return this._data;
   }
 
diff --git a/src/base/io/FileResourceResolver.ts b/src/base/io/FileResourceResolver.ts
index fbf551a6..653ff8ab 100644
--- a/src/base/io/FileResourceResolver.ts
+++ b/src/base/io/FileResourceResolver.ts
@@ -28,13 +28,13 @@ export class FileResourceResolver implements ResourceResolver {
   async resolveDataPartial(
     uri: string,
     maxBytes: number
-  ): Promise<Buffer | null> {
+  ): Promise<Buffer | undefined> {
     if (Uris.isDataUri(uri)) {
       const data = Buffer.from(uri.split(",")[1], "base64");
       return data;
     }
     if (Uris.isAbsoluteUri(uri)) {
-      return null;
+      return undefined;
     }
     const resolved = this.resolveUri(uri);
     try {
@@ -44,22 +44,22 @@ export class FileResourceResolver implements ResourceResolver {
       fs.closeSync(fd);
       return buffer;
     } catch (error) {
-      return null;
+      return undefined;
     }
   }
 
   /** {@inheritDoc ResourceResolver.resolveData} */
-  async resolveData(uri: string): Promise<Buffer | null> {
+  async resolveData(uri: string): Promise<Buffer | undefined> {
     if (Uris.isDataUri(uri)) {
       const data = Buffer.from(uri.split(",")[1], "base64");
       return data;
     }
     if (Uris.isAbsoluteUri(uri)) {
-      return null;
+      return undefined;
     }
     const resolved = this.resolveUri(uri);
     if (!fs.existsSync(resolved)) {
-      return null;
+      return undefined;
     }
     const data = fs.readFileSync(resolved);
     // See https://github.com/nodejs/node/issues/35351
diff --git a/src/base/io/ResourceResolver.ts b/src/base/io/ResourceResolver.ts
index 1347f183..c5936be3 100644
--- a/src/base/io/ResourceResolver.ts
+++ b/src/base/io/ResourceResolver.ts
@@ -27,9 +27,9 @@ export interface ResourceResolver {
    *
    * @param uri - The URI
    * @returns A promise that resolves with the buffer data,
-   * or with `null` if the resource could not be resolved.
+   * or with `undefined` if the resource could not be resolved.
    */
-  resolveData(uri: string): Promise<Buffer | null>;
+  resolveData(uri: string): Promise<Buffer | undefined>;
 
   /**
    * Resolve parts of the data from the given URI.
@@ -37,7 +37,7 @@ export interface ResourceResolver {
    * The given URI may be relative to the base URI for
    * which this instance has been created.
    *
-   * If the resource cannot be resolved, then `null` is
+   * If the resource cannot be resolved, then `undefined` is
    * returned.
    *
    * Otherwise, the returned buffer contains _at least_ the
@@ -46,9 +46,12 @@ export interface ResourceResolver {
    *
    * @param uri - The URI
    * @returns A promise that resolves with the buffer data,
-   * or with `null` if the resource could not be resolved.
+   * or with `undefined` if the resource could not be resolved.
    */
-  resolveDataPartial(uri: string, maxBytes: number): Promise<Buffer | null>;
+  resolveDataPartial(
+    uri: string,
+    maxBytes: number
+  ): Promise<Buffer | undefined>;
 
   /**
    * Derive an instance from this one, with a different base
diff --git a/src/base/io/UnzippingResourceResolver.ts b/src/base/io/UnzippingResourceResolver.ts
index 2676d28b..4fd37cff 100644
--- a/src/base/io/UnzippingResourceResolver.ts
+++ b/src/base/io/UnzippingResourceResolver.ts
@@ -23,10 +23,10 @@ export class UnzippingResourceResolver implements ResourceResolver {
   }
 
   /** {@inheritDoc ResourceResolver.resolveData} */
-  async resolveData(uri: string): Promise<Buffer | null> {
+  async resolveData(uri: string): Promise<Buffer | undefined> {
     const delegateData = await this._delegate.resolveData(uri);
-    if (delegateData === null) {
-      return null;
+    if (delegateData === undefined) {
+      return undefined;
     }
     const isGzipped = Buffers.isGzipped(delegateData);
     if (!isGzipped) {
@@ -40,21 +40,21 @@ export class UnzippingResourceResolver implements ResourceResolver {
   async resolveDataPartial(
     uri: string,
     maxBytes: number
-  ): Promise<Buffer | null> {
+  ): Promise<Buffer | undefined> {
     const partialDelegateData = await this._delegate.resolveDataPartial(
       uri,
       maxBytes
     );
-    if (partialDelegateData === null) {
-      return null;
+    if (partialDelegateData === undefined) {
+      return undefined;
     }
     const isGzipped = Buffers.isGzipped(partialDelegateData);
     if (!isGzipped) {
       return partialDelegateData;
     }
     const fullDelegateData = await this._delegate.resolveData(uri);
-    if (fullDelegateData === null) {
-      return null;
+    if (fullDelegateData === undefined) {
+      return undefined;
     }
     const data = zlib.gunzipSync(fullDelegateData);
     return data;
diff --git a/src/base/spatial/OctreeCoordinates.ts b/src/base/spatial/OctreeCoordinates.ts
index cbdb8c43..a1450a6d 100644
--- a/src/base/spatial/OctreeCoordinates.ts
+++ b/src/base/spatial/OctreeCoordinates.ts
@@ -38,9 +38,9 @@ export class OctreeCoordinates implements TreeCoordinates {
   }
 
   /** {@inheritDoc TreeCoordinates.parent} */
-  parent(): OctreeCoordinates | null {
+  parent(): OctreeCoordinates | undefined {
     if (this._level === 0) {
-      return null;
+      return undefined;
     }
     const pLevel = this._level - 1;
     const px = this._x >> 1;
diff --git a/src/base/spatial/QuadtreeCoordinates.ts b/src/base/spatial/QuadtreeCoordinates.ts
index 118808c8..febbfa5b 100644
--- a/src/base/spatial/QuadtreeCoordinates.ts
+++ b/src/base/spatial/QuadtreeCoordinates.ts
@@ -32,9 +32,9 @@ export class QuadtreeCoordinates implements TreeCoordinates {
   }
 
   /** {@inheritDoc TreeCoordinates.parent} */
-  parent(): QuadtreeCoordinates | null {
+  parent(): QuadtreeCoordinates | undefined {
     if (this._level === 0) {
-      return null;
+      return undefined;
     }
     const pLevel = this._level - 1;
     const px = this._x >> 1;
diff --git a/src/base/spatial/TreeCoordinates.ts b/src/base/spatial/TreeCoordinates.ts
index 3b692e01..f638fce9 100644
--- a/src/base/spatial/TreeCoordinates.ts
+++ b/src/base/spatial/TreeCoordinates.ts
@@ -14,11 +14,11 @@ export interface TreeCoordinates {
 
   /**
    * Returns the parent coordinates of these coordinates,
-   * or `null` if this is the root.
+   * or `undefined` if this is the root.
    *
    * @returns The parent coordinates
    */
-  parent(): TreeCoordinates | null;
+  parent(): TreeCoordinates | undefined;
 
   /**
    * Returns a generator for the child coordinates of these coordinates
diff --git a/src/tilesets/tilesetData/TilesetSourceFs.ts b/src/tilesets/tilesetData/TilesetSourceFs.ts
index 1fc96c0a..0fa7c649 100644
--- a/src/tilesets/tilesetData/TilesetSourceFs.ts
+++ b/src/tilesets/tilesetData/TilesetSourceFs.ts
@@ -60,11 +60,12 @@ export class TilesetSourceFs implements TilesetSource {
     if (!fs.existsSync(fullFileName)) {
       return undefined;
     }
-    const data = fs.readFileSync(fullFileName);
-    if (data === null) {
+    try {
+      const data = fs.readFileSync(fullFileName);
+      return data;
+    } catch (error) {
       return undefined;
     }
-    return data;
   }
 
   /** {@inheritDoc TilesetSource.close} */
diff --git a/src/tilesets/tilesetData/TilesetSourceResourceResolver.ts b/src/tilesets/tilesetData/TilesetSourceResourceResolver.ts
index 0d528044..7ba5fe1f 100644
--- a/src/tilesets/tilesetData/TilesetSourceResourceResolver.ts
+++ b/src/tilesets/tilesetData/TilesetSourceResourceResolver.ts
@@ -25,7 +25,7 @@ export class TilesetSourceResourceResolver implements ResourceResolver {
   }
 
   /** {@inheritDoc ResourceResolver.resolveData} */
-  async resolveData(uri: string): Promise<Buffer | null> {
+  async resolveData(uri: string): Promise<Buffer | undefined> {
     return this.resolveDataInternal(uri);
   }
 
@@ -33,26 +33,26 @@ export class TilesetSourceResourceResolver implements ResourceResolver {
   async resolveDataPartial(
     uri: string,
     maxBytes: number
-  ): Promise<Buffer | null> {
+  ): Promise<Buffer | undefined> {
     const buffer = await this.resolveDataInternal(uri);
     if (!buffer) {
-      return null;
+      return undefined;
     }
     return buffer.subarray(0, maxBytes);
   }
 
-  private async resolveDataInternal(uri: string): Promise<Buffer | null> {
+  private async resolveDataInternal(uri: string): Promise<Buffer | undefined> {
     if (Uris.isDataUri(uri)) {
       const data = Buffer.from(uri.split(",")[1], "base64");
       return data;
     }
     if (Uris.isAbsoluteUri(uri)) {
-      return null;
+      return undefined;
     }
     const localUri = this.resolveUri(uri);
     const value = await this._tilesetSource.getValue(localUri);
     if (!value) {
-      return null;
+      return undefined;
     }
     return value;
   }