diff --git a/lib/srfi/18/threads.c b/lib/srfi/18/threads.c
index 87071a73..b6acffce 100644
--- a/lib/srfi/18/threads.c
+++ b/lib/srfi/18/threads.c
@@ -385,6 +385,8 @@ static sexp sexp_blocker (sexp ctx, sexp self, sexp_sint_t n, sexp portorfd) {
   /* register the fd */
   if (sexp_portp(portorfd))
     fd = sexp_port_fileno(portorfd);
+  else if (sexp_filenop(portorfd))
+    fd = sexp_fileno_fd(portorfd);
   else if (sexp_fixnump(portorfd))
     fd = sexp_unbox_fixnum(portorfd);
   else