cannot use .str() on kernel of Matrix ?

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(with sagemath 10.1 - not checked with previous revision)

A is a matrix defined by:

A=Matrix(GF(2),4,3,[0,1,1,1,0,1,1,1,0,1,1,1])

k is the kernel of A :

k=kernel(A)

Sagemath displays :

Vector space of degree 4 and dimension 1 over Finite Field of size 2
Basis matrix:
[1 1 1 0]

But then, k.str() fails with error message "AttributeError: 'FreeModule_submodule_field_with_category' object has no attribute 'str'".