diff --git a/src/system/libroot/posix/locale/wctype.cpp b/src/system/libroot/posix/locale/wctype.cpp
index d2d225647c..8534aaa28f 100644
--- a/src/system/libroot/posix/locale/wctype.cpp
+++ b/src/system/libroot/posix/locale/wctype.cpp
@@ -123,8 +123,11 @@ iswxdigit(wint_t wc)
 wint_t
 towlower(wint_t wc)
 {
-	if (gLocaleBackend == NULL)
+	if (gLocaleBackend == NULL) {
+		if (wc < 0 || wc > 127)
+			return wc;
 		return tolower(wc);
+	}
 
 	wint_t result = wc;
 	gLocaleBackend->ToWCTrans(wc, _ISlower, result);
@@ -136,8 +139,11 @@ towlower(wint_t wc)
 wint_t
 towupper(wint_t wc)
 {
-	if (gLocaleBackend == NULL)
+	if (gLocaleBackend == NULL) {
+		if (wc < 0 || wc > 127)
+			return wc;
 		return toupper(wc);
+	}
 
 	wint_t result = wc;
 	gLocaleBackend->ToWCTrans(wc, _ISupper, result);