diff --git a/src/org/zaproxy/zap/network/HttpResponseBody.java b/src/org/zaproxy/zap/network/HttpResponseBody.java
index 5d7ffea8983..9d3f459384c 100644
--- a/src/org/zaproxy/zap/network/HttpResponseBody.java
+++ b/src/org/zaproxy/zap/network/HttpResponseBody.java
@@ -90,12 +90,16 @@ protected Charset determineCharset(String contents) {
 			} catch (IllegalArgumentException e) {
 				log.warn("Unable to determine (valid) charset with the (X)HTML meta charset: " + e.getMessage());
 			}
-		} else if (contents.getBytes(StandardCharsets.UTF_8).length == contents.getBytes().length) {
+		} else if (isUtf8String(contents)) {
 			return StandardCharsets.UTF_8;
 		}
 		return null;
 	}
 
+	private static boolean isUtf8String(String string) {
+		return new String(string.getBytes(StandardCharsets.UTF_8), StandardCharsets.UTF_8).length() == string.length();
+	}
+
 	@Override
 	protected String createString(Charset currentCharset) {
 		if (currentCharset != null) {