Python ModRef analysis says that a constant is modified

[khatchad]

Consider the following code:

def g(p1, p2):
    assert p1 == 5 and p2 == 2


def f():
    g(5, p2=2)


f()

The ModRef analysis for Python (com.ibm.wala.cast.python.modref.PythonModRef) is returning that the following locations are "modified" by f():

[[ConstantKey:p2:<PythonLoader,Lstring>]; [Core[Root]]]
[[ConstantKey:p2:<PythonLoader,Lstring>],<field 0>]

But, according to the docs, a ConstantKey is "an instance key which represents a unique, constant object." It would thus seem that instances represented by ConstantKey instance keys can't be returned as modified by the ModRef analysis.