diff --git a/changelog.d/257.bugfix b/changelog.d/257.bugfix
new file mode 100644
index 00000000..9218cb6e
--- /dev/null
+++ b/changelog.d/257.bugfix
@@ -0,0 +1 @@
+Fix .toUpperCase() errors due to the bridge trying to handle unknown deleted messages
diff --git a/src/SlackEventHandler.ts b/src/SlackEventHandler.ts
index df9d02e1..38dec57c 100644
--- a/src/SlackEventHandler.ts
+++ b/src/SlackEventHandler.ts
@@ -120,6 +120,10 @@ export class SlackEventHandler extends BaseSlackHandler {
                 this.main.incCounter("received_messages", {side: "remote"});
                 endTimer({outcome: "dropped"});
                 return;
+            } else if (err.message === "unknown_message") {
+                log.warn(`Ignoring event because we couldn't find a referred to message`);
+                endTimer({outcome: "dropped"});
+                return;
             } else if (err.message === "unknown_event") {
                 endTimer({outcome: "dropped"});
             } else {
@@ -213,6 +217,8 @@ export class SlackEventHandler extends BaseSlackHandler {
                 const botClient = this.main.botIntent.getClient();
                 return botClient.redactEvent(originalEvent.roomId, originalEvent.eventId);
             }
+            // If we don't have the event
+            throw Error("unknown_message");
         } else if (msg.subtype === "message_replied") {
             // Slack sends us one of these as well as a normal message event
             // when using RTM, so we ignore it.