diff --git a/src/ir/lower.zig b/src/ir/lower.zig
index ed82ab9..25513b3 100644
--- a/src/ir/lower.zig
+++ b/src/ir/lower.zig
@@ -8730,6 +8730,29 @@ pub const Lowering = struct {
         const saved_defer_base = self.func_defer_base;
         const saved_block_terminated = self.block_terminated;
         const saved_target = self.target_type;
+        // Pack-fn mono state is lexical to the pack-fn body. A generic
+        // function called from inside a pack-fn mono (e.g.
+        // `build(args: []Type, $ret: Type)` invoked from
+        // `probe(..$args) { build($args, void) }`) must not inherit the
+        // caller's pack maps — `lowerFieldAccess`'s `<pack_name>.len`
+        // intercept would otherwise constant-fold the callee's
+        // same-named param to whichever shape triggered the first mono
+        // and bake the wrong arity into the cached IR. Same shape of
+        // fix as `lazyLowerFunction` (issue-0048, commit 0ede097).
+        const saved_pan = self.pack_arg_nodes;
+        const saved_ppc = self.pack_param_count;
+        const saved_pat = self.pack_arg_types;
+        const saved_iri = self.inline_return_target;
+        self.pack_arg_nodes = null;
+        self.pack_param_count = null;
+        self.pack_arg_types = null;
+        self.inline_return_target = null;
+        defer {
+            self.pack_arg_nodes = saved_pan;
+            self.pack_param_count = saved_ppc;
+            self.pack_arg_types = saved_pat;
+            self.inline_return_target = saved_iri;
+        }
         self.func_defer_base = self.defer_stack.items.len;
         self.block_terminated = false;