diff --git a/lib/libc/tests/string/wcsnlen_test.c b/lib/libc/tests/string/wcsnlen_test.c
index ec11938b19a9..07870e3a5ca0 100644
--- a/lib/libc/tests/string/wcsnlen_test.c
+++ b/lib/libc/tests/string/wcsnlen_test.c
@@ -65,7 +65,7 @@ test_wcsnlen(const wchar_t *s)
 	for (i = 0; i <= 1; i++) {
 		for (bufsize = 0; bufsize <= size + 10; bufsize++) {
 			s1 = makebuf(bufsize * sizeof(wchar_t), i);
-			wmemcpy(s1, s, bufsize);
+			wmemcpy(s1, s, bufsize <= size ? bufsize : size);
 			len = (size > bufsize) ? bufsize : size - 1;
 			ATF_CHECK(wcsnlen(s1, bufsize) == len);
 		}