diff --git a/cogs/mod.py b/cogs/mod.py
index 8aaefc8..6b7badd 100644
--- a/cogs/mod.py
+++ b/cogs/mod.py
@@ -1693,7 +1693,7 @@ def joined_before(member, *, _other=args.joined_before):
             return await ctx.send('Aborting.')
 
         count = 0
-        for member in members.values():
+        for member in list(members.values()):
             try:
                 await ctx.guild.ban(member, reason=reason)
             except discord.HTTPException: