diff --git a/libsrc/cbm/workingscreenaddr.s b/libsrc/cbm/workingscreenaddr.s
index a2780dce6..7c2d1f488 100644
--- a/libsrc/cbm/workingscreenaddr.s
+++ b/libsrc/cbm/workingscreenaddr.s
@@ -13,6 +13,16 @@ _cbm_set_working_screen:
         tax
         and #3
         bne @error
+        cpx #$10    ; Screen addr must not be set to $1000-$1fff nor $9000-$9fff
+        bcc @ok_1000
+        cpx #$1f
+        bcc @error
+    @ok_1000:
+        cpx #$90    ; Screen addr must not be set to $1000-$1fff nor $9000-$9fff
+        bcc @ok_9000
+        cpx #$9f
+        bcc @error
+    @ok_9000:
         stx SCREEN_HI
         sec
         jsr PLOT    ; Get cursor position ...