diff --git a/protocols/inspircd.py b/protocols/inspircd.py
index a48cbc9..97bf542 100644
--- a/protocols/inspircd.py
+++ b/protocols/inspircd.py
@@ -467,6 +467,13 @@ class InspIRCdProtocol(TS6BaseProtocol):
         namelist = []
         for user in userlist:
             modeprefix, user = user.split(',', 1)
+
+            # Don't crash when we get an invalid UID.
+            if user not in self.irc.users:
+                log.debug('(%s) handle_fjoin: tried to introduce user %s not in our user list, ignoring...',
+                          self.irc.name, user)
+                continue
+
             namelist.append(user)
             self.irc.users[user].channels.add(channel)
             if their_ts <= our_ts:
diff --git a/protocols/ts6.py b/protocols/ts6.py
index 2fadabf..f0d4ec6 100644
--- a/protocols/ts6.py
+++ b/protocols/ts6.py
@@ -435,6 +435,13 @@ class TS6Protocol(TS6BaseProtocol):
             finalprefix = ''
             assert user, 'Failed to get the UID from %r; our regex needs updating?' % userpair
             log.debug('(%s) handle_sjoin: got modeprefix %r for user %r', self.irc.name, modeprefix, user)
+
+            # Don't crash when we get an invalid UID.
+            if user not in self.irc.users:
+                log.debug('(%s) handle_sjoin: tried to introduce user %s not in our user list, ignoring...',
+                          self.irc.name, user)
+                continue
+
             for m in modeprefix:
                 # Iterate over the mapping of prefix chars to prefixes, and
                 # find the characters that match.