diff --git a/src/gtk/dialog.cpp b/src/gtk/dialog.cpp
index c70813d557..c61fbca515 100644
--- a/src/gtk/dialog.cpp
+++ b/src/gtk/dialog.cpp
@@ -98,11 +98,8 @@ void wxDialog::SetModal( bool WXUNUSED(flag) )
 
 int wxDialog::ShowModal()
 {
-    if (IsModal())
-    {
-       wxFAIL_MSG( wxT("wxDialog:ShowModal called twice") );
-       return GetReturnCode();
-    }
+    wxASSERT_MSG( !IsModal(), "ShowModal() can't be called twice" );
+    wxASSERT_MSG( !IsShown(), "ShowModal() can't be called after Show()" );
 
     // release the mouse if it's currently captured as the window having it
     // will be disabled when this dialog is shown -- but will still keep the
diff --git a/src/msw/dialog.cpp b/src/msw/dialog.cpp
index 9d28a6e039..b0703f2291 100644
--- a/src/msw/dialog.cpp
+++ b/src/msw/dialog.cpp
@@ -283,7 +283,8 @@ void wxDialog::Raise()
 // show dialog modally
 int wxDialog::ShowModal()
 {
-    wxASSERT_MSG( !IsModal(), _T("wxDialog::ShowModal() reentered?") );
+    wxASSERT_MSG(!IsShown(),
+                 _T("ShowModal() can't be called for an already shown dialog"));
 
     m_endModalCalled = false;