diff --git a/appservice/util.go b/appservice/util.go
index 4175fb0..160f492 100644
--- a/appservice/util.go
+++ b/appservice/util.go
@@ -2,7 +2,7 @@ package appservice
 
 import (
 	"fmt"
-	"strings"
+	"unicode"
 
 	log "github.com/sirupsen/logrus"
 
@@ -44,8 +44,15 @@ func userMxId(protocol string, id UserID) string {
 }
 
 func safeStringForId(in string) string {
-	id2 := strings.ReplaceAll(in, "#", "")
-	id2 = strings.ReplaceAll(id2, "@", "__")
-	id2 = strings.ReplaceAll(id2, ":", "_")
+	id2 := ""
+	for _, c := range in {
+		if c == '@' {
+			id2 += "__"
+		} else if c == ':' {
+			id2 += "_"
+		} else if unicode.IsDigit(c) || unicode.IsLetter(c) {
+			id2 += string(c)
+		}
+	}
 	return id2
 }