Vyper is a pythonic Smart Contract Language for the Ethereum virtual machine. Starting in version 0.3.8 and prior to version 0.4.0b1, when looping over a "range" of the form "range(start, start + N)", if "start" is negative, the execution will always revert. This issue is caused by an incorrect assertion inserted by the code generation of the range "stmt.parse_For_range()". The issue arises when "start" is signed, instead of using "sle", "le" is used and "start" is interpreted as an unsigned integer for the comparison. If it is a negative number, its 255th bit is set to "1" and is hence interpreted as a very large unsigned integer making the assertion always fail. Any contract having a "range(start, start + N)" where "start" is a signed integer with the possibility for "start" to be negative is affected. If a call goes through the loop while supplying a negative "start" the execution will revert. Version 0.4.0b1 fixes the issue.