Question: How to use animate with matrix plot?

Hi
I would like to matrixplot consecutive powers of a matrix. As the name of the matrix is not always the same I wrote a simple "embedded" procedure WM to do this.
This produre works well when called directly, but I don't understand how to make it work when called from animate
 

WM := P -> n -> plots:-matrixplot(P^n, heights=histogram):

U := Matrix(2$2, [0.8, 0.2, 0.4, 0.6]);
WM(U)(1);  #ok whatever the value of the argument of W(U)

plots:-animate(WM(U), [n], n=1..2);  # produces an error

# even this simpler command doesn't work
plots:-animate(plots:-matrixplot, [U^n], n=1..2)  # same error as above

Can you help me to fix this?

TIA

PS: I firstly defined WM this way

WM := proc(P, n)  plots:-matrixplot(P^n, heights=histogram) end proc:

but I met difficulties to tell animate that only n was the parameter to change