diff --git a/src/main/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposer.java b/src/main/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposer.java
index a3f4a4530c..7f4abb0a42 100644
--- a/src/main/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposer.java
+++ b/src/main/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposer.java
@@ -30,6 +30,7 @@
 import com.google.api.generator.engine.ast.NewObjectExpr;
 import com.google.api.generator.engine.ast.ScopeNode;
 import com.google.api.generator.engine.ast.Statement;
+import com.google.api.generator.engine.ast.StringObjectValue;
 import com.google.api.generator.engine.ast.ThisObjectValue;
 import com.google.api.generator.engine.ast.TypeNode;
 import com.google.api.generator.engine.ast.ValueExpr;
@@ -186,7 +187,6 @@ private static MethodDefinition createAddResponseMethod() {
   }
 
   private static MethodDefinition createSetResponsesMethod(Service service) {
-    // TODO(miraleung): Re-instantiate the fields here.
     VariableExpr responsesArgVarExpr =
         VariableExpr.withVariable(
             Variable.builder()
@@ -490,6 +490,14 @@ private static Statement createHandleObjectStatement(
     }
 
     TypeNode exceptionType = TypeNode.withReference(ConcreteReference.withClazz(Exception.class));
+    Expr newExceptionExpr =
+        NewObjectExpr.builder()
+            .setType(
+                TypeNode.withReference(ConcreteReference.withClazz(IllegalArgumentException.class)))
+            .setArguments(
+                Arrays.asList(
+                    ValueExpr.withValue(StringObjectValue.withValue("Unrecognized response type"))))
+            .build();
 
     return IfStatement.builder()
         .setConditionExpr(
@@ -524,6 +532,7 @@ private static Statement createHandleObjectStatement(
                     MethodInvocationExpr.builder()
                         .setMethodName("onError")
                         .setExprReferenceExpr(responseObserverVarExpr)
+                        .setArguments(Arrays.asList(newExceptionExpr))
                         .build())))
         .build();
   }
diff --git a/src/test/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposerTest.java b/src/test/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposerTest.java
index e2f56d13a6..8fe7478590 100644
--- a/src/test/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposerTest.java
+++ b/src/test/java/com/google/api/generator/gapic/composer/MockServiceImplClassComposerTest.java
@@ -123,7 +123,8 @@ public void generateServiceClasses() {
           + "    } else if (response instanceof Exception) {\n"
           + "      responseObserver.onError(((Exception) response));\n"
           + "    } else {\n"
-          + "      responseObserver.onError();\n"
+          + "      responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "    }\n"
           + "  }\n"
           + "\n"
@@ -138,7 +139,8 @@ public void generateServiceClasses() {
           + "    } else if (response instanceof Exception) {\n"
           + "      responseObserver.onError(((Exception) response));\n"
           + "    } else {\n"
-          + "      responseObserver.onError();\n"
+          + "      responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "    }\n"
           + "  }\n"
           + "\n"
@@ -156,7 +158,8 @@ public void generateServiceClasses() {
           + "            } else if (response instanceof Exception) {\n"
           + "              responseObserver.onError(((Exception) response));\n"
           + "            } else {\n"
-          + "              responseObserver.onError();\n"
+          + "              responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "            }\n"
           + "          }\n"
           + "\n"
@@ -187,7 +190,8 @@ public void generateServiceClasses() {
           + "            } else if (response instanceof Exception) {\n"
           + "              responseObserver.onError(((Exception) response));\n"
           + "            } else {\n"
-          + "              responseObserver.onError();\n"
+          + "              responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "            }\n"
           + "          }\n"
           + "\n"
@@ -218,7 +222,8 @@ public void generateServiceClasses() {
           + "            } else if (response instanceof Exception) {\n"
           + "              responseObserver.onError(((Exception) response));\n"
           + "            } else {\n"
-          + "              responseObserver.onError();\n"
+          + "              responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "            }\n"
           + "          }\n"
           + "\n"
@@ -247,7 +252,8 @@ public void generateServiceClasses() {
           + "    } else if (response instanceof Exception) {\n"
           + "      responseObserver.onError(((Exception) response));\n"
           + "    } else {\n"
-          + "      responseObserver.onError();\n"
+          + "      responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "    }\n"
           + "  }\n"
           + "\n"
@@ -262,7 +268,8 @@ public void generateServiceClasses() {
           + "    } else if (response instanceof Exception) {\n"
           + "      responseObserver.onError(((Exception) response));\n"
           + "    } else {\n"
-          + "      responseObserver.onError();\n"
+          + "      responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "    }\n"
           + "  }\n"
           + "\n"
@@ -277,7 +284,8 @@ public void generateServiceClasses() {
           + "    } else if (response instanceof Exception) {\n"
           + "      responseObserver.onError(((Exception) response));\n"
           + "    } else {\n"
-          + "      responseObserver.onError();\n"
+          + "      responseObserver.onError(new "
+          + "IllegalArgumentException(\"Unrecognized response type\"));\n"
           + "    }\n"
           + "  }\n"
           + "}\n";