--- a/comm/mailnews/base/src/MboxMsgInputStream.cpp.old	2024-09-29 13:27:36.274947489 +0200
+++ b/comm/mailnews/base/src/MboxMsgInputStream.cpp	2024-09-29 13:27:47.384867824 +0200
@@ -241,6 +241,7 @@
       default:
         MOZ_ASSERT_UNREACHABLE();  // should not happen
     }
+    return data;
   }
 
   // We're expecting a new message to start, or an EOF.