diff --git a/src/libsync/abstractnetworkjob.h b/src/libsync/abstractnetworkjob.h
index 63391e672e..04dadb49ec 100644
--- a/src/libsync/abstractnetworkjob.h
+++ b/src/libsync/abstractnetworkjob.h
@@ -77,7 +77,7 @@ public:
     bool timedOut() const { return _timedout; }
 
     /** Returns an error message, if any. */
-    QString errorString() const;
+    virtual QString errorString() const;
 
     /** Like errorString, but also checking the reply body for information.
      *
diff --git a/src/libsync/propagateupload.h b/src/libsync/propagateupload.h
index 1a547a3b68..b53cc65204 100644
--- a/src/libsync/propagateupload.h
+++ b/src/libsync/propagateupload.h
@@ -131,7 +131,7 @@ public:
         return _device;
     }
 
-    QString errorString()
+    QString errorString() const override
     {
         return _errorString.isEmpty() ? AbstractNetworkJob::errorString() : _errorString;
     }