diff --git a/authentik/providers/oauth2/models.py b/authentik/providers/oauth2/models.py
index baf2f45f4..a85b1869a 100644
--- a/authentik/providers/oauth2/models.py
+++ b/authentik/providers/oauth2/models.py
@@ -487,7 +487,7 @@ class RefreshToken(SerializerModel, ExpiringModel, BaseGrantModel):
         if self.provider.sub_mode == SubModes.HASHED_USER_ID:
             sub = user.uid
         elif self.provider.sub_mode == SubModes.USER_ID:
-            sub = user.pk
+            sub = str(user.pk)
         elif self.provider.sub_mode == SubModes.USER_EMAIL:
             sub = user.email
         elif self.provider.sub_mode == SubModes.USER_USERNAME: