diff --git a/google/google.go b/google/google.go
index 4d287ca..0594699 100644
--- a/google/google.go
+++ b/google/google.go
@@ -186,7 +186,7 @@ func (f *credentialsFile) tokenSource(ctx context.Context, params CredentialsPar
 		}
 		return cfg.TokenSource(ctx)
 	case impersonatedServiceAccount:
-		if f.SourceCredentials == nil {
+		if f.ServiceAccountImpersonationURL != "" && f.SourceCredentials == nil {
 			return nil, errors.New("missing 'source_credentials' field in credentials")
 		}