diff --git a/src/json-enc/json-enc.js b/src/json-enc/json-enc.js
index 6dff5d4..b51230d 100644
--- a/src/json-enc/json-enc.js
+++ b/src/json-enc/json-enc.js
@@ -1,12 +1,28 @@
-htmx.defineExtension('json-enc', {
-  onEvent: function(name, evt) {
-    if (name === 'htmx:configRequest') {
-      evt.detail.headers['Content-Type'] = 'application/json'
+(function() {
+  let api
+  htmx.defineExtension('json-enc', {
+    init: function(apiRef) {
+      api = apiRef
+    },
+
+    onEvent: function(name, evt) {
+      if (name === 'htmx:configRequest') {
+        evt.detail.headers['Content-Type'] = 'application/json'
+      }
+    },
+
+    encodeParameters: function(xhr, parameters, elt) {
+      xhr.overrideMimeType('text/json')
+
+      const vals = api.getExpressionVars(elt)
+      const object = {}
+      parameters.forEach(function(value, key) {
+        // FormData encodes values as strings, restore hx-vals/hx-vars with their initial types
+        object[key] = Object.hasOwn(vals, key) ? vals[key] : value
+      })
+
+      return (JSON.stringify(object))
     }
-  },
+  })
 
-  encodeParameters: function(xhr, parameters, elt) {
-    xhr.overrideMimeType('text/json')
-    return (JSON.stringify(parameters))
-  }
-})
+})()
diff --git a/src/json-enc/test/ext/json-enc.js b/src/json-enc/test/ext/json-enc.js
index f12d9e6..dd06d88 100644
--- a/src/json-enc/test/ext/json-enc.js
+++ b/src/json-enc/test/ext/json-enc.js
@@ -135,4 +135,23 @@ describe('json-enc extension', function() {
     this.server.lastRequest.response.should.equal('{"passwordok":true}')
     htmx.config.methodsThatUseUrlParams = defaults
   })
+
+  it('handles hx-vals properly', function() {
+    var values = {}
+    this.server.respondWith('POST', '/test', function(xhr) {
+      values = JSON.parse(xhr.requestBody)
+      xhr.respond(200, {}, 'clicked')
+    })
+
+    make(`<form hx-ext="json-enc" hx-post="/test" hx-vals="js:{'obj': {'x': 123}, 'number': 5000, 'numberString': '5000'}">
+             <button id="btn" type="submit">Submit</button>
+          </form>`)
+    var btn = byId('btn')
+    btn.click()
+    this.server.respond()
+
+    values.number.should.equal(5000)
+    values.numberString.should.equal('5000')
+    chai.assert.deepEqual(values.obj, {'x': 123})
+  })
 })