HP Prime: Unexpected "poly1" after cross product

[parisse]

poly1[ is a delimiter for a list representing a univariate polynomial (with coefficients in decreasing order). If you add lists that have different sizes, then they can not be considered as vectors (at least in the same vector space), and addition will mean univariate polynomials addition.
Using poly1[ instead of [ as a list left delimiter is a way to disambiguate some operations like *, if you multiply [1,2]*[3,4] by default dot product is used and it returns a real, if you multiply poly1[1,2]*[3,4] polynomial multiplication will be used.