diff --git a/libcxx/include/__support/musl/xlocale.h b/libcxx/include/__support/musl/xlocale.h
--- a/libcxx/include/__support/musl/xlocale.h
+++ b/libcxx/include/__support/musl/xlocale.h
@@ -37,7 +37,8 @@
   return ::wcstoll(__nptr, __endptr, __base);
 }
 
-inline _LIBCPP_HIDE_FROM_ABI_C long long wcstoull_l(const wchar_t* __nptr, wchar_t** __endptr, int __base, locale_t) {
+inline _LIBCPP_HIDE_FROM_ABI_C unsigned long long
+wcstoull_l(const wchar_t* __nptr, wchar_t** __endptr, int __base, locale_t) {
   return ::wcstoull(__nptr, __endptr, __base);
 }