diff --git a/test/data/event/onbeforeunload.html b/test/data/event/onbeforeunload.html
index 11ad1964a..30053967a 100644
--- a/test/data/event/onbeforeunload.html
+++ b/test/data/event/onbeforeunload.html
@@ -4,17 +4,18 @@
 	<script>
 		function report( event ) {
 			var payload = {
+				source: "jQuery onbeforeunload iframe test",
 				event: event.type
 			};
-			return parent.postMessage( JSON.stringify(payload), "*" );
+			return parent.postMessage( JSON.stringify( payload ), "*" );
 		}
 
 		jQuery( window ).on( "beforeunload", function( event ) {
 			report( event );
-		}).on( "load", function( event ) {
-			setTimeout(function() {
+		} ).on( "load", function( event ) {
+			setTimeout( function() {
 				window.location.reload();
-			}, 50);
-		});
+			}, 50 );
+		} );
 	</script>
 </html>
diff --git a/test/unit/event.js b/test/unit/event.js
index 38a805b37..ac309d22d 100644
--- a/test/unit/event.js
+++ b/test/unit/event.js
@@ -1448,13 +1448,22 @@ QUnit[ /(ipad|iphone|ipod)/i.test( navigator.userAgent ) ? "skip" : "test" ](
 	var done = assert.async();
 
 	window.onmessage = function( event ) {
-		var payload = JSON.parse( event.data );
+		try {
+			var payload = JSON.parse( event.data );
 
-		assert.ok( payload.event, "beforeunload", "beforeunload event" );
+			// Ignore unrelated messages
+			if ( payload.source === "jQuery onbeforeunload iframe test" ) {
+				assert.ok( payload.event, "beforeunload", "beforeunload event" );
 
-		iframe.remove();
-		window.onmessage = null;
-		done();
+				iframe.remove();
+				window.onmessage = null;
+				done();
+			}
+		} catch ( e ) {
+
+			// Messages may come from other sources, like browser extensions;
+			// some may not be valid JSONs and thus cannot be `JSON.parse`d.
+		}
 	};
 
 	iframe.appendTo( "#qunit-fixture" );