diff --git a/src/html/helpwnd.cpp b/src/html/helpwnd.cpp
index 51e081117d..e9aac75806 100644
--- a/src/html/helpwnd.cpp
+++ b/src/html/helpwnd.cpp
@@ -1503,7 +1503,9 @@ void wxHtmlHelpWindow::OnToolbar(wxCommandEvent& event)
                 {
                     m_BookmarksNames.RemoveAt(pos);
                     m_BookmarksPages.RemoveAt(pos);
-                    m_Bookmarks->Delete(m_Bookmarks->GetSelection());
+                    pos = m_Bookmarks->GetSelection();
+                    wxASSERT_MSG( pos != wxNOT_FOUND , wxT("Unknown bookmark position") ) ;
+                    m_Bookmarks->Delete((unsigned int)pos);
                 }
             }
             break;