When serializing a PSBT (BIP-174, Partially Signed Bitcoin Transaction), I cannot find any information if the keys should be ordered when serializing the PSBT?

Under the "Test Vectors" section for the BIP-174 in the last example it is only mentioned that:

A combiner which orders keys lexicographically must produce the following PSBT[...]

But it does nowhere say that it is mandatory to order the keys. What is expected?

I am implementing test cases for a PSBT library and would like to know this to decide if I should compare the serialized PSBTs (where ordering does matter) or the objects themselves (regardless of ordering of keys).