diff --git a/expat/tests/common.c b/expat/tests/common.c
index b158385f..b2537d0d 100644
--- a/expat/tests/common.c
+++ b/expat/tests/common.c
@@ -303,7 +303,14 @@ duff_reallocator(void *ptr, size_t size) {
   return realloc(ptr, size);
 }
 
-// Portable remake of strndup(3) for C99; does not care about space efficiency
+// Portable remake of strnlen(3) for C99
+static size_t
+portable_strnlen(const char *s, size_t maxlen) {
+  const char *const end = (const char *)memchr(s, '\0', maxlen);
+  return (end == NULL) ? maxlen : (size_t)(end - s);
+}
+
+// Portable remake of strndup(3) for C99
 char *
 portable_strndup(const char *s, size_t n) {
   if ((s == NULL) || (n == SIZE_MAX)) {
@@ -311,6 +318,8 @@ portable_strndup(const char *s, size_t n) {
     return NULL;
   }
 
+  n = portable_strnlen(s, n);
+
   char *const buffer = (char *)malloc(n + 1);
   if (buffer == NULL) {
     errno = ENOMEM;