diff --git a/desk.acc/eyes.s b/desk.acc/eyes.s
index 5eeacbf..3be4c69 100644
--- a/desk.acc/eyes.s
+++ b/desk.acc/eyes.s
@@ -36,6 +36,8 @@ entry:
         sta     ALTZPON
         lda     LCBANK1
         lda     LCBANK1
+
+        ;; back to main for exit
         sta     RAMRDOFF
         sta     RAMWRTOFF
         rts
@@ -191,7 +193,11 @@ grow_box_bitmap:
         lda     LCBANK1
         lda     LCBANK1
 
+        ;; Don't let MGTK smash zero page
         MGTK_CALL MGTK::SetZP1, preserve_zp_params
+        lda     #0
+        sta     SHIFT_SIGN_EXT  ; Must zero before using FP ops
+
         MGTK_CALL MGTK::OpenWindow, winfo
         jsr     draw_window
         MGTK_CALL MGTK::FlushEvents
diff --git a/inc/apple2.inc b/inc/apple2.inc
index 71c2151..19745f8 100644
--- a/inc/apple2.inc
+++ b/inc/apple2.inc
@@ -71,6 +71,7 @@ LINNUM          := $50          ; Line number, unsigned word
 ERRFLG          := $D8
 TXTPTR          := $B8
 FAC             := $9D
+SHIFT_SIGN_EXT  := $A4          ; Zero before using FP ops
 
 ;;; Entry Points
 GIVAYF          := $E2F2        ; FAC from signed integer in (Y,A)