Try using a definition like so above your inputs…
def tolist(obj1):
if hasattr(obj1,“iter”): return obj1
else: return [obj1]
And for the input beams write the following…
beams = tolist(UnwrapElement(IN[0]))
This is what I use for inputs to ensure they are an iterable object. Very common point of failure in writing python nodes.
(I can’t seem to format the code on my mobile, but indented code after the first line naturally)