diff --git a/src/stdlib/wcstoumax.c b/src/stdlib/wcstoumax.c
index 456ddedddfb805bb841bfa9220fc4a9b1f93f4bd..86528ef15715be4952bb950ac6941594d3c84783 100644
--- a/src/stdlib/wcstoumax.c
+++ b/src/stdlib/wcstoumax.c
@@ -34,7 +34,7 @@ uintmax_t wcstoumax(const wchar_t *s, wchar_t **p, int base)
 
 	/* Convert to normal char string so we can use strtoumax */
 	buf[0] = sign;
-	if (wcstombs(buf+!!sign, s, sizeof buf-1) < 0) return 0;
+	if (wcstombs(buf+!!sign, s, sizeof buf-1) == -1) return 0;
 	buf[sizeof buf-1]=0;
 
 	/* Compute final position */