diff --git a/include/wx/gtk/toplevel.h b/include/wx/gtk/toplevel.h
index 88abfd03cf..b057a4a0ce 100644
--- a/include/wx/gtk/toplevel.h
+++ b/include/wx/gtk/toplevel.h
@@ -143,7 +143,7 @@ public:
 
     virtual void SetMinSize(const wxSize& minSize) wxOVERRIDE;
 
-    virtual void WXSetInitialFittingClientSize(int flags) wxOVERRIDE;
+    virtual void WXSetInitialFittingClientSize(int flags, wxSizer* sizer = NULL) wxOVERRIDE;
 
 private:
     // Flags to call WXSetInitialFittingClientSize() with if != 0.
diff --git a/src/gtk/toplevel.cpp b/src/gtk/toplevel.cpp
index 6060a7a3ac..6b01391e3e 100644
--- a/src/gtk/toplevel.cpp
+++ b/src/gtk/toplevel.cpp
@@ -1567,10 +1567,11 @@ void wxTopLevelWindowGTK::SetMinSize(const wxSize& minSize)
     m_pendingFittingClientSizeFlags &= ~wxSIZE_SET_MIN;
 }
 
-void wxTopLevelWindowGTK::WXSetInitialFittingClientSize(int flags)
+void
+wxTopLevelWindowGTK::WXSetInitialFittingClientSize(int flags, wxSizer* sizer)
 {
     // In any case, update the size immediately.
-    wxTopLevelWindowBase::WXSetInitialFittingClientSize(flags);
+    wxTopLevelWindowBase::WXSetInitialFittingClientSize(flags, sizer);
 
     // But if we're not shown yet, the fitting size may be wrong because GTK
     // style cache hasn't been updated yet and we need to do it again when the