diff --git a/lib/roken/roken.h.in b/lib/roken/roken.h.in
index 37f963a12..5528d9425 100644
--- a/lib/roken/roken.h.in
+++ b/lib/roken/roken.h.in
@@ -203,7 +203,7 @@ int vasnprintf (char **ret, size_t max_sz, const char *format, va_list ap)
 char * strdup(const char *old);
 #endif
 
-#ifndef HAVE_STRNDUP
+#if !defined(HAVE_STRNDUP) || defined(NEED_STRNDUP_PROTO)
 char * strndup(const char *old, size_t sz);
 #endif