diff --git a/google/api_core/gapic_v1/method.py b/google/api_core/gapic_v1/method.py
index 50d4356..9c4cf03 100644
--- a/google/api_core/gapic_v1/method.py
+++ b/google/api_core/gapic_v1/method.py
@@ -127,7 +127,12 @@
 
         # Add the user agent metadata to the call.
         if self._metadata is not None:
-            metadata = list(kwargs.get('metadata', []))
+            metadata = kwargs.get('metadata', [])
+            # Due to the nature of invocation, None should be treated the same
+            # as not specified.
+            if metadata is None:
+                metadata = []
+            metadata = list(metadata)
             metadata.extend(self._metadata)
             kwargs['metadata'] = metadata
 
diff --git a/tests/unit/gapic/test_method.py b/tests/unit/gapic/test_method.py
index 4592cb1..4ce4e8e 100644
--- a/tests/unit/gapic/test_method.py
+++ b/tests/unit/gapic/test_method.py
@@ -94,6 +94,19 @@
     assert ('a', 'b') in metadata
 
 
+def test_invoke_wrapped_method_with_metadata_as_none():
+    method = mock.Mock(spec=['__call__'])
+
+    wrapped_method = google.api_core.gapic_v1.method.wrap_method(method)
+
+    wrapped_method(mock.sentinel.request, metadata=None)
+
+    method.assert_called_once_with(mock.sentinel.request, metadata=mock.ANY)
+    metadata = method.call_args[1]['metadata']
+    # Metadata should have just one items: the client info metadata.
+    assert len(metadata) == 1
+
+
 @mock.patch('time.sleep')
 def test_wrap_method_with_default_retry_and_timeout(unusued_sleep):
     method = mock.Mock(