diff --git a/irc/modes.go b/irc/modes.go
index 7cd297f0..375831eb 100644
--- a/irc/modes.go
+++ b/irc/modes.go
@@ -54,9 +54,15 @@ func (client *Client) applyUserModeChanges(force bool, changes modes.ModeChanges
 			}
 			var masks []sno.Mask
 			if change.Op == modes.Add || change.Op == modes.Remove {
+				var newArg string
 				for _, char := range change.Arg {
-					masks = append(masks, sno.Mask(char))
+					mask := sno.Mask(char)
+					if sno.ValidMasks[mask] {
+						masks = append(masks, mask)
+						newArg += string(char)
+					}
 				}
+				change.Arg = newArg
 			}
 			if change.Op == modes.Add {
 				client.server.snomasks.AddMasks(client, masks...)
diff --git a/irc/sno/constants.go b/irc/sno/constants.go
index 697348a9..5449962e 100644
--- a/irc/sno/constants.go
+++ b/irc/sno/constants.go
@@ -35,4 +35,18 @@ var (
 		LocalAccounts:      "ACCOUNT",
 		LocalXline:         "XLINE",
 	}
+
+	// ValidMasks contains the snomasks that we support.
+	ValidMasks = map[Mask]bool{
+		LocalAccouncements: true,
+		LocalConnects:      true,
+		LocalChannels:      true,
+		LocalKills:         true,
+		LocalNicks:         true,
+		LocalOpers:         true,
+		LocalQuits:         true,
+		Stats:              true,
+		LocalAccounts:      true,
+		LocalXline:         true,
+	}
 )
diff --git a/irc/snomanager.go b/irc/snomanager.go
index 848a87e7..38ab8d6c 100644
--- a/irc/snomanager.go
+++ b/irc/snomanager.go
@@ -27,6 +27,11 @@ func (m *SnoManager) AddMasks(client *Client, masks ...sno.Mask) {
 	defer m.sendListMutex.Unlock()
 
 	for _, mask := range masks {
+		// confirm mask is valid
+		if !sno.ValidMasks[mask] {
+			continue
+		}
+
 		currentClientList := m.sendLists[mask]
 
 		if currentClientList == nil {