diff --git a/moto/iam/models.py b/moto/iam/models.py
index c210caad1..5110a012e 100644
--- a/moto/iam/models.py
+++ b/moto/iam/models.py
@@ -446,6 +446,6 @@ class IAMBackend(BaseBackend):
         report = 'user,arn,user_creation_time,password_enabled,password_last_used,password_last_changed,password_next_rotation,mfa_active,access_key_1_active,access_key_1_last_rotated,access_key_2_active,access_key_2_last_rotated,cert_1_active,cert_1_last_rotated,cert_2_active,cert_2_last_rotated\n'
         for user in self.users:
             report += self.users[user].to_csv()
-        return base64.b64encode(report)
+        return base64.b64encode(report.encode('ascii'))
 
 iam_backend = IAMBackend()