diff --git a/runtime/bin/vmservice/server.dart b/runtime/bin/vmservice/server.dart
index fb272607f56..a0446ad222a 100644
--- a/runtime/bin/vmservice/server.dart
+++ b/runtime/bin/vmservice/server.dart
@@ -296,6 +296,12 @@ class Server {
       return;
     }
 
+    if (assets == null) {
+      request.response.headers.contentType = ContentType.TEXT;
+      request.response.write("This VM was built without the Observatory UI.");
+      request.response.close();
+      return;
+    }
     Asset asset = assets[path];
     if (asset != null) {
       // Serving up a static asset (e.g. .css, .html, .png).
diff --git a/runtime/lib/vmservice_patch.dart b/runtime/lib/vmservice_patch.dart
index fac7e6d67ad..2618e1b6a32 100644
--- a/runtime/lib/vmservice_patch.dart
+++ b/runtime/lib/vmservice_patch.dart
@@ -7,12 +7,12 @@ class Asset {
   /// Call to request assets from the embedder.
   @patch
   static HashMap<String, Asset> request() {
-    HashMap<String, Asset> assets = new HashMap<String, Asset>();
     Uint8List tarBytes = _requestAssets();
     if (tarBytes == null) {
-      return assets;
+      return null;
     }
     List assetList = _decodeAssets(tarBytes);
+    HashMap<String, Asset> assets = new HashMap<String, Asset>();
     for (int i = 0; i < assetList.length; i += 2) {
       var a = new Asset(assetList[i], assetList[i + 1]);
       assets[a.name] = a;