Why it is not a Huffman code

Question

I have been given several examples I the aim is to explain why it is not a Huffman code. So, for instance, the first one was:

$\{00,01,10,110\}$

This code is not Huffman becuase it has just one codeword of maximum length whereas there should be two as a minimum.

Next, the one I hava a problem in proving:

$\{01,10\}$

Why this code is not a Huffman code?