diff --git a/src/generic/collpaneg.cpp b/src/generic/collpaneg.cpp
index dc9874f98c..d0fd7bfdb2 100644
--- a/src/generic/collpaneg.cpp
+++ b/src/generic/collpaneg.cpp
@@ -190,6 +190,8 @@ void wxGenericCollapsiblePane::Collapse(bool collapse)
     if ( IsCollapsed() == collapse )
         return;
 
+    InvalidateBestSize();
+
     // update our state
     m_pPane->Show(!collapse);