diff --git a/src/libexpr/nix_api_value.cc b/src/libexpr/nix_api_value.cc
index f58500367..ba36fdc3c 100644
--- a/src/libexpr/nix_api_value.cc
+++ b/src/libexpr/nix_api_value.cc
@@ -42,7 +42,11 @@ PrimOp *nix_alloc_primop(nix_c_context *context, PrimOpFun fun, int arity,
 #ifdef HAVE_BOEHMGC
         (GC)
 #endif
-            nix::PrimOp{.name = name, .args = {}, .doc = doc, .fun = fun2};
+            nix::PrimOp{.name = name,
+                        .args = {},
+                        .arity = (size_t)arity,
+                        .doc = doc,
+                        .fun = fun2};
     if (args)
       for (size_t i = 0; args[i]; i++)
         p->args.emplace_back(*args);