Summary

The decimals function is optional in the initial ERC20 and might fail for old tokens that do not implement it.

Tools Used

Manual Review

Recommendations

Here is an example of how to fix the issue:

https://github.com/boringcrypto/BoringSolidity/blob/c73ed73afa9273fbce93095ef177513191782254/contracts/libraries/BoringERC20.sol#L49-L55