diff --git a/posix.vapi b/posix.vapi
index 43e0c88..935b368 100644
--- a/posix.vapi
+++ b/posix.vapi
@@ -1153,7 +1153,7 @@ namespace Posix {
        public int mkostemp (string template, int flags);
 
        [CCode (cheader_filename = "stdlib.h")]
-       public string? realpath (string path, uint8[]? resolved_path = null);
+       public string? realpath (string path, [CCode (array_length=false)] uint8[]? resolved_path = null);
 
        [CCode (cheader_filename = "stdlib.h")]
        public int posix_openpt (int flags);