diff --git a/third_party/lisp/ironclad.nix b/third_party/lisp/ironclad.nix
index 9bdf87114..fa860d4d0 100644
--- a/third_party/lisp/ironclad.nix
+++ b/third_party/lisp/ironclad.nix
@@ -16,6 +16,7 @@ in depot.nix.buildLisp.library {
   deps = with depot.third_party.lisp; [
     (bundled "asdf")
     (bundled "sb-rotate-byte")
+    (bundled "sb-posix")
     alexandria
     bordeaux-threads
     nibbles