diff --git a/source/session.ts b/source/session.ts
index 202a75b..2749c5b 100644
--- a/source/session.ts
+++ b/source/session.ts
@@ -618,7 +618,7 @@ export class Session {
             content: (reason["cause"] as string) || reason.message,
           });
         }
-        throw new Error("Unknown error");
+        throw new Error("Unknown error: " + JSON.stringify(reason));
       }
 
       const response = await fetchResponse.json();