diff --git a/include/wx/msw/button.h b/include/wx/msw/button.h
index 1f5ad27c9b..ee1d197e9d 100644
--- a/include/wx/msw/button.h
+++ b/include/wx/msw/button.h
@@ -88,8 +88,10 @@ protected:
     virtual void DoSetBitmapMargins(wxCoord x, wxCoord y);
     virtual void DoSetBitmapPosition(wxDirection dir);
 
-    // Increases the passed in size if necessary to account for the
-    // button image, if any
+    // Increases the passed in size to account for the button image.
+    //
+    // Should only be called if we do have a button, i.e. if m_imageData is
+    // non-NULL.
     void AdjustForBitmapSize(wxSize& size) const;
 
     class wxButtonImageData *m_imageData;
diff --git a/src/msw/button.cpp b/src/msw/button.cpp
index 240e2e9813..cabac36506 100644
--- a/src/msw/button.cpp
+++ b/src/msw/button.cpp
@@ -531,8 +531,7 @@ void wxButton::SetLabel(const wxString& label)
 
 void wxButton::AdjustForBitmapSize(wxSize &size) const
 {
-    if ( !m_imageData )
-        return;
+    wxCHECK_RET( m_imageData, wxT("shouldn't be called if no image") );
 
     // account for the bitmap size
     const wxSize sizeBmp = m_imageData->GetBitmap(State_Normal).GetSize();
@@ -614,6 +613,8 @@ wxSize wxButton::DoGetBestSize() const
     {
         AdjustForBitmapSize(size);
 
+        // The best size has changed so even if it had been already cached by
+        // ComputeBestSize() call above we still need to update it.
         CacheBestSize(size);
     }