diff --git a/google/auth/impersonated_credentials.py b/google/auth/impersonated_credentials.py
index c2351e7..84df484 100644
--- a/google/auth/impersonated_credentials.py
+++ b/google/auth/impersonated_credentials.py
@@ -88,6 +88,7 @@
 
     response = request(url=iam_endpoint, method="POST", headers=headers, body=body)
 
+    # support both string and bytes type response.data
     response_body = (
         response.data.decode("utf-8")
         if hasattr(response.data, "decode")