TypeError: '<' not supported between instances of 'int' and 'bytes'

[psimatis]

Hello

I get the following error trying to insert to the B+Tree:
TypeError: '<' not supported between instances of 'int' and 'bytes'

The command is the following:
tree.insert(bytes(enVal, encoding='utf8'), bytes(val,encoding='utf8'))

enVal and val are strings.

What am I doing wrong?