Incorrect obfuscation with no imports

[nbkolchin]

Input:

# Copyright comment
"""Common exceptions"""

class CannotRun(Exception):
    """Thrown from Base classes when running is impossible"""
    pass

Obfuscated: (--obfuscate-builtins --replacement-length=11)

class CannotRun(kfgyztSaQBc):
kfgyztSaQBc=Exception
 pass
# Created by pyminifier (https://github.com/liftoff/pyminifier)

Removing first line comment or adding dummy import produces correct output

import os
mxUpuXFLbBM=Exception
class CannotRun(mxUpuXFLbBM):
 pass
# Created by pyminifier (https://github.com/liftoff/pyminifier)