diff --git a/src/utils.ts b/src/utils.ts
index 1cfdfe8b..643322c1 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -622,6 +622,10 @@ const unknownUsers = new Set();
 const unknownMembers = new Set();
 
 export function resolveUserId(bot: Client, value: string) {
+  if (value == null) {
+    return null;
+  }
+
   // A user mention?
   const mentionMatch = value.match(/^<@!?(\d+)>$/);
   if (mentionMatch) {