diff --git a/src/__tests__/use.ts b/src/__tests__/use.ts
index 7d8d0c4a..8455ac34 100644
--- a/src/__tests__/use.ts
+++ b/src/__tests__/use.ts
@@ -504,6 +504,27 @@ for (const { tServer, skipUWS, startTServer } of tServers) {
       });
     });
 
+    it('should handle and limit internal server errors that are not instances of `Error`', async () => {
+      const { url } = await startTServer({
+        onConnect: () => {
+          throw 'i am exactly 124 characters long i am exactly 124 characters long i am exactly 124 characters long i am exactly 124 characte';
+        },
+      });
+
+      const client = await createTClient(url);
+      client.ws.send(
+        stringifyMessage<MessageType.ConnectionInit>({
+          type: MessageType.ConnectionInit,
+        }),
+      );
+
+      await client.waitForClose((event) => {
+        expect(event.code).toBe(CloseCode.InternalServerError);
+        expect(event.reason).toBe('Internal server error');
+        expect(event.wasClean).toBeTruthy(); // because the server reported the error
+      });
+    });
+
     describe('Keep-Alive', () => {
       it('should dispatch pings after the timeout has passed', async (done) => {
         const { url } = await startTServer(undefined, 50);
diff --git a/src/use/@fastify/websocket.ts b/src/use/@fastify/websocket.ts
index e77e1d07..4602f507 100644
--- a/src/use/@fastify/websocket.ts
+++ b/src/use/@fastify/websocket.ts
@@ -79,7 +79,10 @@ export function makeHandler<
               CloseCode.InternalServerError,
               isProd
                 ? 'Internal server error'
-                : limitCloseReason(err.message, 'Internal server error'),
+                : limitCloseReason(
+                    err instanceof Error ? err.message : String(err),
+                    'Internal server error',
+                  ),
             );
           } catch (err) {
             firstErr = firstErr ?? err;
@@ -104,7 +107,10 @@ export function makeHandler<
         CloseCode.InternalServerError,
         isProd
           ? 'Internal server error'
-          : limitCloseReason(err.message, 'Internal server error'),
+          : limitCloseReason(
+              err instanceof Error ? err.message : String(err),
+              'Internal server error',
+            ),
       );
     }
 
@@ -161,7 +167,10 @@ export function makeHandler<
                 CloseCode.InternalServerError,
                 isProd
                   ? 'Internal server error'
-                  : limitCloseReason(err.message, 'Internal server error'),
+                  : limitCloseReason(
+                      err instanceof Error ? err.message : String(err),
+                      'Internal server error',
+                    ),
               );
             }
           }),
diff --git a/src/use/uWebSockets.ts b/src/use/uWebSockets.ts
index d175b3fc..aa67afe6 100644
--- a/src/use/uWebSockets.ts
+++ b/src/use/uWebSockets.ts
@@ -205,7 +205,10 @@ export function makeBehavior<
           CloseCode.InternalServerError,
           isProd
             ? 'Internal server error'
-            : limitCloseReason(err.message, 'Internal server error'),
+            : limitCloseReason(
+                err instanceof Error ? err.message : String(err),
+                'Internal server error',
+              ),
         );
       }
     },
diff --git a/src/use/ws.ts b/src/use/ws.ts
index d1cedaf0..f991edfa 100644
--- a/src/use/ws.ts
+++ b/src/use/ws.ts
@@ -73,7 +73,10 @@ export function useServer<
           CloseCode.InternalServerError,
           isProd
             ? 'Internal server error'
-            : limitCloseReason(err.message, 'Internal server error'),
+            : limitCloseReason(
+                err instanceof Error ? err.message : String(err),
+                'Internal server error',
+              ),
         );
       } catch (err) {
         firstErr = firstErr ?? err;
@@ -94,7 +97,10 @@ export function useServer<
         CloseCode.InternalServerError,
         isProd
           ? 'Internal server error'
-          : limitCloseReason(err.message, 'Internal server error'),
+          : limitCloseReason(
+              err instanceof Error ? err.message : String(err),
+              'Internal server error',
+            ),
       );
     });
 
@@ -146,7 +152,10 @@ export function useServer<
                 CloseCode.InternalServerError,
                 isProd
                   ? 'Internal server error'
-                  : limitCloseReason(err.message, 'Internal server error'),
+                  : limitCloseReason(
+                      err instanceof Error ? err.message : String(err),
+                      'Internal server error',
+                    ),
               );
             }
           }),