Submitted by gpersoon, also found by hickuphh3 and pauliax
The function _requireYieldSource of the contract SwappableYieldSource has a state variable: isInvalidYieldSource
You would expect isInvalidYieldSource == true would mean the yield source is invalid
However in the source code  isInvalidYieldSource == true mean the yield source is valid.
This is confusing for readers and future maintainers. Future maintainers could easily make a mistake and thus introduce vulnerabilities.
Recommend changing isInvalidYieldSource to isValidYieldSource
PierrickGT (PoolTogether) confirmed and patched:
