diff --git a/include/wx/window.h b/include/wx/window.h
index 794d7a0a15..393d71ba49 100644
--- a/include/wx/window.h
+++ b/include/wx/window.h
@@ -1571,7 +1571,11 @@ public:
     // that we really need to use is not known until the window is actually
     // shown, as is the case for TLWs with recent GTK versions, as it will
     // update the size again when it does become known, if necessary.
-    virtual void WXSetInitialFittingClientSize(int flags);
+    //
+    // The optional sizer argument can be passed to use the given sizer for
+    // laying out the window, which is useful if this function is called before
+    // SetSizer(). By default the window sizer is used.
+    virtual void WXSetInitialFittingClientSize(int flags, wxSizer* sizer = NULL);
 
         // get the handle of the window for the underlying window system: this
         // is only used for wxWin itself or for user code which wants to call
diff --git a/src/common/sizer.cpp b/src/common/sizer.cpp
index 755ba7fd3e..3b18ac910f 100644
--- a/src/common/sizer.cpp
+++ b/src/common/sizer.cpp
@@ -1079,7 +1079,7 @@ wxSize wxSizer::Fit( wxWindow *window )
     wxCHECK_MSG( window, wxDefaultSize, "window can't be NULL" );
 
     // set client size
-    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT);
+    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT, this);
 
     // return entire size
     return window->GetSize();
@@ -1119,7 +1119,8 @@ void wxSizer::SetSizeHints( wxWindow *window )
 {
     // Preserve the window's max size hints, but set the
     // lower bound according to the sizer calculations.
-    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT | wxSIZE_SET_MIN);
+    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT | wxSIZE_SET_MIN,
+                                          this);
 }
 
 #if WXWIN_COMPATIBILITY_2_8
diff --git a/src/common/wincmn.cpp b/src/common/wincmn.cpp
index bb8127670c..a851b371ce 100644
--- a/src/common/wincmn.cpp
+++ b/src/common/wincmn.cpp
@@ -999,11 +999,17 @@ wxSize wxWindowBase::WindowToClientSize(const wxSize& size) const
                   size.y == -1 ? -1 : size.y - diff.y);
 }
 
-void wxWindowBase::WXSetInitialFittingClientSize(int flags)
+void wxWindowBase::WXSetInitialFittingClientSize(int flags, wxSizer* sizer)
 {
-    wxSizer* const sizer = GetSizer();
+    // Use the window sizer by default.
     if ( !sizer )
-        return;
+    {
+        sizer = GetSizer();
+
+        // If there is none, we can't compute the fitting size.
+        if ( !sizer )
+            return;
+    }
 
     const wxSize
         size = sizer->ComputeFittingClientSize(static_cast<wxWindow *>(this));