diff --git a/skfem/experimental/autodiff/__init__.py b/skfem/experimental/autodiff/__init__.py
index c1d525f0..224449b1 100644
--- a/skfem/experimental/autodiff/__init__.py
+++ b/skfem/experimental/autodiff/__init__.py
@@ -114,9 +114,12 @@ def assemble(self, basis, x=None, **kwargs):
         dx = basis.dx
 
         defaults = basis.default_parameters()
+        # turn defaults into JaxDiscreteField to avoid np.ndarray
+        # to jnp.ndarray promotion issues
         w = FormExtraParams({
             **{
-                k: JaxDiscreteField(*tuple(jnp.asarray(x) for x in defaults[k].astuple))
+                k: JaxDiscreteField(*tuple(jnp.asarray(x)
+                                           for x in defaults[k].astuple))
                 for k in defaults
             },
             **self._normalize_asm_kwargs(kwargs, basis),