diff --git a/google/auth/iam.py b/google/auth/iam.py
index 5e88a04..5d63dc5 100644
--- a/google/auth/iam.py
+++ b/google/auth/iam.py
@@ -79,7 +79,7 @@
 
         if response.status != http_client.OK:
             raise exceptions.TransportError(
-                "Error calling the IAM signBytes API: {}".format(response.data)
+                "Error calling the IAM signBlob API: {}".format(response.data)
             )
 
         return json.loads(response.data.decode("utf-8"))