Create theano custom operation with [tt.dmatrix, tt.dmatrix] input

Questions
3

After replacing tens_a and tens_b by tt.dmatrix, I run into another problem.
The operation returns [None, None] instead of the result that I expect.

Test:

import numpy as np
import theano.tensor as tt
import theano


class MyOp(tt.Op):
    itypes = [tt.dmatrix, tt.dmatrix]
    otypes = [tt.dmatrix, tt.dmatrix]

    def __init__(self):
        pass

    def perform(self, node, inputs, outputs):
        mat_a, mat_b = inputs
        outputs[0] = 2 * mat_a
        outputs[1] = 3 * mat_b



def main():

    tens_a = tt.dmatrix('a')
    tens_b = tt.dmatrix('b')
    funct = MyOp()(tens_a, tens_b)
    f = theano.function([tens_a, tens_b], funct)

    dtype = np.float64
    mat_a = np.ones((3, 3), dtype=dtype)
    mat_b = np.ones((3, 3), dtype=dtype)
    print(f(mat_a, mat_b))


main()

Result:

[None, None]