diff --git a/google/api_core/operation.py b/google/api_core/operation.py
index a66e4a50..b17f753b 100644
--- a/google/api_core/operation.py
+++ b/google/api_core/operation.py
@@ -140,11 +140,11 @@ def _set_result_from_operation(self):
                 )
                 self.set_exception(exception)
             else:
-                # Some APIs set `done: true`, with an empty response.
-                # Set the result to an empty message of the expected
-                # result type.
-                # https://google.aip.dev/151
-                self.set_result(self._result_type())
+                exception = exceptions.GoogleAPICallError(
+                    "Unexpected state: Long-running operation had neither "
+                    "response nor error set."
+                )
+                self.set_exception(exception)
 
     def _refresh_and_update(self, retry=polling.DEFAULT_RETRY):
         """Refresh the operation and update the result if needed.
diff --git a/google/api_core/operation_async.py b/google/api_core/operation_async.py
index 17624d62..6bae8654 100644
--- a/google/api_core/operation_async.py
+++ b/google/api_core/operation_async.py
@@ -136,11 +136,11 @@ def _set_result_from_operation(self):
                 )
                 self.set_exception(exception)
             else:
-                # Some APIs set `done: true`, with an empty response.
-                # Set the result to an empty message of the expected
-                # result type.
-                # https://google.aip.dev/151
-                self.set_result(self._result_type())
+                exception = exceptions.GoogleAPICallError(
+                    "Unexpected state: Long-running operation had neither "
+                    "response nor error set."
+                )
+                self.set_exception(exception)
 
     async def _refresh_and_update(self, retry=async_future.DEFAULT_RETRY):
         """Refresh the operation and update the result if needed.
diff --git a/tests/asyncio/test_operation_async.py b/tests/asyncio/test_operation_async.py
index 886b1c8d..26ad7cef 100644
--- a/tests/asyncio/test_operation_async.py
+++ b/tests/asyncio/test_operation_async.py
@@ -158,7 +158,7 @@ async def test_exception():
 
 @mock.patch("asyncio.sleep", autospec=True)
 @pytest.mark.asyncio
-async def test_done_with_no_error_or_response(unused_sleep):
+async def test_unexpected_result(unused_sleep):
     responses = [
         make_operation_proto(),
         # Second operation response is done, but has not error or response.
@@ -166,9 +166,9 @@ async def test_done_with_no_error_or_response(unused_sleep):
     ]
     future, _, _ = make_operation_future(responses)
 
-    result = await future.result()
+    exception = await future.exception()
 
-    assert isinstance(result, struct_pb2.Struct)
+    assert "Unexpected state" in "{!r}".format(exception)
 
 
 def test_from_gapic():
diff --git a/tests/unit/test_operation.py b/tests/unit/test_operation.py
index bafff8b0..22e23bc3 100644
--- a/tests/unit/test_operation.py
+++ b/tests/unit/test_operation.py
@@ -169,7 +169,7 @@ def test_exception_with_error_code():
     assert isinstance(exception, exceptions.NotFound)
 
 
-def test_done_with_no_error_or_response():
+def test_unexpected_result():
     responses = [
         make_operation_proto(),
         # Second operation response is done, but has not error or response.
@@ -177,7 +177,9 @@ def test_done_with_no_error_or_response():
     ]
     future, _, _ = make_operation_future(responses)
 
-    assert isinstance(future.result(), struct_pb2.Struct)
+    exception = future.exception()
+
+    assert "Unexpected state" in "{!r}".format(exception)
 
 
 def test__refresh_http():