diff --git a/fasthtml/core.py b/fasthtml/core.py
index 46b4d1b9..b084ee2d 100644
--- a/fasthtml/core.py
+++ b/fasthtml/core.py
@@ -1025,7 +1025,7 @@ def add_sig_param(f, name, typ=NoneType, kind=Parameter.KEYWORD_ONLY, default=Pa
     sig = signature(f)
     if name in sig.parameters: return f
     kw = {} if default is Parameter.empty else {'default': default}
-    new_params = list(sig.parameters.values()) + [Parameter(name, kind, **kw)]
+    new_params = list(sig.parameters.values()) + [Parameter(name, kind, annotation=typ, **kw)]
     f.__signature__ = sig.replace(parameters=new_params)
     f.__annotations__[name] = typ
     return f
diff --git a/nbs/api/00_core.ipynb b/nbs/api/00_core.ipynb
index d8827e07..6323cfab 100644
--- a/nbs/api/00_core.ipynb
+++ b/nbs/api/00_core.ipynb
@@ -4534,7 +4534,7 @@
     "    sig = signature(f)\n",
     "    if name in sig.parameters: return f\n",
     "    kw = {} if default is Parameter.empty else {'default': default}\n",
-    "    new_params = list(sig.parameters.values()) + [Parameter(name, kind, **kw)]\n",
+    "    new_params = list(sig.parameters.values()) + [Parameter(name, kind, annotation=typ, **kw)]\n",
     "    f.__signature__ = sig.replace(parameters=new_params)\n",
     "    f.__annotations__[name] = typ\n",
     "    return f"