googleapis
diff --git a/google/api_core/grpc_helpers.py b/google/api_core/grpc_helpers.py
@@ -216,7 +216,7 @@ def _create_composite_credentials(
     else:
         credentials, _ = google.auth.default(scopes=scopes)
 
-    if quota_project_id:
+    if quota_project_id and isinstance(credentials, google.auth.credentials.CredentialsWithQuotaProject):
         credentials = credentials.with_quota_project(quota_project_id)
 
     request = google.auth.transport.requests.Request()

diff --git a/setup.py b/setup.py
@@ -31,7 +31,7 @@
 dependencies = [
     "googleapis-common-protos >= 1.6.0, < 2.0dev",
     "protobuf >= 3.12.0",
-    "google-auth >= 1.19.1, < 2.0dev",
+    "google-auth >= 1.21.1, < 2.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
     "setuptools >= 34.0.0",
     "six >= 1.10.0",

diff --git a/tests/asyncio/test_grpc_helpers_async.py b/tests/asyncio/test_grpc_helpers_async.py
@@ -369,7 +369,7 @@ def test_create_channel_explicit_with_quota_project(grpc_secure_channel, composi
     target = "example.com:443"
     composite_creds = composite_creds_call.return_value
 
-    credentials = mock.create_autospec(google.auth.credentials.Credentials, instance=True)
+    credentials = mock.create_autospec(google.auth.credentials.CredentialsWithQuotaProject, instance=True)
 
     channel = grpc_helpers_async.create_channel(
         target, credentials=credentials, quota_project_id="project-foo"

diff --git a/tests/unit/test_grpc_helpers.py b/tests/unit/test_grpc_helpers.py
@@ -341,7 +341,7 @@ def test_create_channel_explicit_with_quota_project(grpc_secure_channel, composi
     target = "example.com:443"
     composite_creds = composite_creds_call.return_value
 
-    credentials = mock.create_autospec(google.auth.credentials.Credentials, instance=True)
+    credentials = mock.create_autospec(google.auth.credentials.CredentialsWithQuotaProject, instance=True)
 
     channel = grpc_helpers.create_channel(
         target,