diff --git a/libsrc/common/tolower.s b/libsrc/common/tolower.s
index fa03146b0..0c67199a3 100644
--- a/libsrc/common/tolower.s
+++ b/libsrc/common/tolower.s
@@ -8,12 +8,14 @@
 	.import		__ctype, __cdiff
 
 _tolower:
+        cpx     #$00            ; Outside valid range?
+        bne     L9              ; If so, return the argument unchanged
        	tay			; Get C into Y
 	lda	__ctype,y	; Get character classification
 	lsr	a
 	lsr	a		; Get bit 1 (upper case char) into carry
 	tya			; Get char back into A
        	bcc    	L9		; Jump if no upper case char
-   	sbc	__cdiff		; make lower case char (carry already set)
-L9:    	rts	  		; CC are set
+   	sbc	__cdiff	       	; Make lower case char (carry already set)
+L9:    	rts
 
diff --git a/libsrc/common/toupper.s b/libsrc/common/toupper.s
index ab7711cb2..5561eb7d4 100644
--- a/libsrc/common/toupper.s
+++ b/libsrc/common/toupper.s
@@ -8,6 +8,8 @@
 	.import		__ctype, __cdiff
 
 _toupper:
+        cpx     #$00            ; Outside valid range?
+        bne     L9              ; If so, return the argument unchanged
 	tay			; Get c into Y
 	lda	__ctype,y	; Get character classification
 	lsr	a		; Get bit 0 (lower char) into carry