GridMap GDScript Can't work

I am Japanese.
I am learning English now.

Godot 3.x

This GDScript code can’t work.

func _redraw() → void:
gridmap.clear()
for j in range(_maze_height):
for i in range(_maze_width):
if _array.getv(i,j) == array_wall:
gridmap.set_cell_item(i, 1, j, 0)

Why it can’t work?