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int() ignores everything after a null byte when reporting an error message.
Here you can see an example of how this manifests, and why could be a problem.
Python 2.7.6 (default, Nov 10 2013, 19:24:18) [MSC v.1500 32 bit(Intel)] on win32
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>>> int('a')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'a'
>>> int('0円a')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: ''
>>> int('abc0円def')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'abc'

It looks like the issue was already fixed in Python 3:
Python 3.5.0a0 (default:61d9aa8be445, May 20 2014, 16:03:51) 
>>> int('a')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'a'
>>> int('0円a')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '\x00a'
>>> int('abc\x00def')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'abc\x00def'
And the last one:
>>> int('0円')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '\x00'

See issue 16741. This is fixed in 3.3, but 2.7 intentionally keeps the old behavior.