Error: stack underflow

I see this message "Error: stack underflow" when a loader menu is presented.
It seems that it comes from ficl.  This is on a quite recent (< 2 weeks) head.
How can I debug this problem?

I have one local modification to forth files, but I'm not sure if the problem is
caused by it or by something in my boot configuration files.

diff --git a/sys/boot/forth/menu-commands.4th b/sys/boot/forth/menu-commands.4th
index 9adf30a46b661..813cbf12e9655 100644
--- a/sys/boot/forth/menu-commands.4th
+++ b/sys/boot/forth/menu-commands.4th
_at__at_ -243,6 +243,21 _at__at_ also menu-namespace also menu-command-helpers
 	TRUE \ loop menu again
 ;

+: toggle_gui ( N -- N TRUE )
+	toggle_menuitem
+	menu-redraw
+
+	\ Now we're going to make the change effective
+
+	dup toggle_stateN _at_ 0= if
+		s" inhibit_gui" unsetenv
+	else
+		s" set inhibit_gui=1" evaluate
+	then
+
+	TRUE \ loop menu again
+;
+
 \
 \ Escape to Prompt
 \
diff --git a/sys/boot/forth/menu.rc b/sys/boot/forth/menu.rc
index 3c7de7138b8ad..ddeccc9679fea 100644
--- a/sys/boot/forth/menu.rc
+++ b/sys/boot/forth/menu.rc
_at__at_ -126,6 +126,13 _at__at_ set optionsmenu_keycode[6]=118
 set optionsansi_caption[6]="^[1mV^[merbose..... ^[34;1mOff^[m"
 set optionstoggled_ansi[6]="^[1mV^[merbose..... ^[32;7mOn^[m"

+set optionsmenu_caption[7]="Inhibit [G]UI. off"
+set optionstoggled_text[7]="Inhibit [G]UI. On"
+set optionsmenu_command[7]="toggle_gui"
+set optionsmenu_keycode[7]=103
+set optionsansi_caption[7]="Inhibit GUI. Off"
+set optionstoggled_ansi[7]="Inhibit GUI. On"
+
 \
 \ BOOT ENVIRONMENT MENU
 \


-- 
Andriy Gapon