diff --git a/src/shared/message_handler.js b/src/shared/message_handler.js
index ee4122649..88c565775 100644
--- a/src/shared/message_handler.js
+++ b/src/shared/message_handler.js
@@ -223,6 +223,7 @@ MessageHandler.prototype = {
       },
 
       cancel: (reason) => {
+        assert(reason instanceof Error, 'cancel must have a valid reason');
         let cancelCapability = createPromiseCapability();
         this.streamControllers[streamId].cancelCall = cancelCapability;
         this.streamControllers[streamId].isClosed = true;
@@ -287,6 +288,7 @@ MessageHandler.prototype = {
       },
 
       error(reason) {
+        assert(reason instanceof Error, 'error must have a valid reason');
         if (this.isCancelled) {
           return;
         }
diff --git a/test/unit/message_handler_spec.js b/test/unit/message_handler_spec.js
index 908391e10..2eaee0030 100644
--- a/test/unit/message_handler_spec.js
+++ b/test/unit/message_handler_spec.js
@@ -13,7 +13,9 @@
  * limitations under the License.
  */
 
-import { AbortException, createPromiseCapability } from '../../src/shared/util';
+import {
+  AbortException, createPromiseCapability, UnknownErrorException
+} from '../../src/shared/util';
 import { LoopbackPort } from '../../src/display/api';
 import { MessageHandler } from '../../src/shared/message_handler';
 
@@ -149,7 +151,7 @@ describe('message_handler', function () {
           return sink.ready;
         }).then(() => {
           log += 'e';
-          sink.error('error');
+          sink.error(new Error('should not read when errored'));
         });
       });
       let messageHandler1 = new MessageHandler('main', 'worker', port);
@@ -171,7 +173,8 @@ describe('message_handler', function () {
         return reader.read();
       }).catch((reason) => {
         expect(log).toEqual('01pe');
-        expect(reason).toEqual('error');
+        expect(reason instanceof UnknownErrorException).toEqual(true);
+        expect(reason.message).toEqual('should not read when errored');
         done();
       });
     });