How to concatenate several images given a list

Question

I have a certain scrip that spits a procedurally generated list of lists of strings of what would my tiles be in a square grid:

grid = [['grass', 'grass', 'trees'], 
['flowers', 'grass', 'grass'], 
['grass', 'flowers', 'grass']]

Now I can't seem to find a way to use these names in order to build an image using images of each tile

grass = cv2.imread('grass.png')
flowers = cv2.imread('flowers.png')
trees = cv2.imread('trees.png')

I am sorry I am quite new here, this is what I tried to implement:

image_grid = None
image_row = None

for row in grid:
    for item in row:
        if image_row is None:
            image_row = eval(item)
        else:
            image_row = cv2.hconcat([image_row, eval(item)])

    if image_grid is None:
        image_grid = image_row
    else:
        image_grid = cv2.vconcat([image_grid, image_row])

cv2.imshow('grid', image_grid)
cv2.waitKey()

But it returns cv2.error: OpenCV(4.5.5) D:\a\opencv-python\opencv-python\opencv\modules\core\src\matrix_operations.cpp:67: error: (-215:Assertion failed)