diff --git a/interface/wx/stream.h b/interface/wx/stream.h
index 4f3bb18a45..e5fb42d385 100644
--- a/interface/wx/stream.h
+++ b/interface/wx/stream.h
@@ -680,7 +680,7 @@ protected:
         reached or an error occurred (in this last case the internal @c m_lasterror
         variable should be set accordingly as well).
     */
-    size_t OnSysRead(void* buffer, size_t bufsize);
+    size_t OnSysRead(void* buffer, size_t bufsize) = 0;
 };