fix return type

Author: mfschubert

src/fmmax/fmm.py

@@ -1318,7 +1318,7 @@ def _validate_and_broadcast(
     wavelength_dtype = jnp.promote_types(wavelength.dtype, permittivities[0].real.dtype)
     permittivity_dtype = jnp.promote_types(wavelength_dtype, jnp.complex64)
     wavelength = wavelength.astype(wavelength_dtype)
-    permittivities = [p.astype(permittivity_dtype) for p in permittivities]
+    permittivities = tuple(p.astype(permittivity_dtype) for p in permittivities)
 
     return (
         wavelength,