diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 12c95eb..5db1fa2 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -310,9 +310,13 @@
         self._issuer = issuer
         self._subject = subject
         self._audience = audience
-        self._additional_claims = additional_claims or {}
         self._token_lifetime = token_lifetime
 
+        if additional_claims is not None:
+            self._additional_claims = additional_claims
+        else:
+            self._additional_claims = {}
+
     @classmethod
     def from_service_account_info(cls, info, **kwargs):
         """Creates a Credentials instance from parsed service account info.