diff --git a/src/storage/implementation/observer.ts b/src/storage/implementation/observer.ts
index c781727b184..3b240f35609 100644
--- a/src/storage/implementation/observer.ts
+++ b/src/storage/implementation/observer.ts
@@ -46,10 +46,14 @@ export class Observer<T> {
       this.error = opt_error || null;
       this.complete = opt_complete || null;
     } else {
-      const observer = nextOrObserver as {[name: string]: null};
-      this.next = observer['next'] as (NextFn<T> | null);
-      this.error = observer['error'] as (ErrorFn | null);
-      this.complete = observer['complete'] as (CompleteFn | null);
+      const observer = nextOrObserver as {
+        next?: NextFn<T> | null;
+        error?: ErrorFn | null;
+        complete?: CompleteFn | null;
+      };
+      this.next = observer.next || null;
+      this.error = observer.error || null;
+      this.complete = observer.complete || null;
     }
   }
 }
diff --git a/tests/storage/browser/task_test.ts b/tests/storage/browser/task_test.ts
index 95ca72e4fcf..a0d429c82f3 100644
--- a/tests/storage/browser/task_test.ts
+++ b/tests/storage/browser/task_test.ts
@@ -231,6 +231,18 @@ describe("Firebase Storage > Upload Task", () => {
           });
     });
   });
+  it("Works properly with an observer missing the 'next' method", () => {
+    const authWrapper = authWrapperWithHandler(fakeServerHandler());
+    const task = new UploadTask(
+        {} as Reference, authWrapper, testLocation, mappings, smallBlob);
+    return fbsPromise.make<void>((resolve, reject) => {
+      task.on(
+          TaskEvent.STATE_CHANGED, {
+            error: err => { assert.fail('Unexpected upload failure'); },
+            complete: () => { resolve(null); }
+          });
+      });
+  });
 
   function runNormalUploadTest(blob: FbsBlob): Promise<void> {
     const authWrapper = authWrapperWithHandler(fakeServerHandler());