Fixes #15470

src/bun.js/api/server.zig

@@ -5765,6 +5765,10 @@ pub fn NewServer(comptime NamespaceType: type, comptime ssl_enabled_: bool, comp
                 return JSValue.jsNumber(0);
             }
 
+            if (this.config.websocket == null or this.app == null) {
+                return JSValue.jsNumber(0);
+            }
+
             return JSValue.jsNumber((this.app.?.numSubscribers(topic.slice())));
         }
 

test/js/bun/websocket/websocket-server.test.ts

@@ -825,3 +825,13 @@ async function connect(server: Server): Promise<void> {
   clients.push(client);
   await promise;
 }
+
+it("you can call server.subscriberCount() when its not a websocket server", async () => {
+  const server = serve({
+    port: 0,
+    fetch(request, server) {
+      return new Response();
+    },
+  });
+  expect(server.subscriberCount("boop")).toBe(0);
+});