Seg Fault on NULL strlen

[Steve & Patti Getzinger <psatg bluemarble net>]

I am attempting to set sensitivity based on entries in 3 entry boxes. So 
basically the first is easy. I am hitting a func on changed of entry1 in 
the func I am doing an

> void
> on_entry1_changed                (GtkEditable     *editable,
>                                         gpointer         user_data)
> {
>   if(strlen(gtk_editable_get_chars(GTK_EDITABLE(entry1),0,-1)) > 7)
>         {
>                   
> if(strlen(gtk_editable_get_chars(GTK_EDITABLE(entry2),0,-1)) > 0)

>                         {
>                         gtk_widget_set_sensitive (button1, TRUE);
>                         }
>         }else{
>                 gtk_widget_set_sensitive (button1, FALSE);
>         }
> }

With this config it seg faults with:

> (gdiald:16598): GLib-GObject-WARNING **: invalid cast from (NULL) 
> pointer to `GtkEditable'
>
> (gdiald:16598): Gtk-CRITICAL **: file gtkeditable.c: line 125 
> (gtk_editable_get_chars): assertion `GTK_IS_EDITABLE (editable)' failed
> Segmentation fault


I am assuming strlen returns NULL if there has been no entry made in 
entry2. How do I get around this one?

TIA
Steve