Trying to get luminance_alphamask to work

lum_mask.py

from direct.showbase.ShowBase import ShowBase
from panda3d.core import Texture, CardMaker,  TransparencyAttrib
import numpy as np 

#Create texture
texSize = 512
frequency = 8
def sin3d(X, freq = 1):
    return np.sin(X*freq)
def sin8bit(X, freq = 1):
    sin_float = sin3d(X, freq=freq)
    sin_pos = (sin_float+1)*127.5; #from 0-255
    return np.asarray(sin_pos, dtype = np.uint8)
x = np.linspace(0, 2*np.pi, texSize)  
y = np.linspace(0, 2*np.pi, texSize)
X, Y = np.meshgrid(x,y)
sinTex = sin8bit(X, freq = frequency);

#Create mask
bandRadius = 30
leftMask = 255*np.ones((texSize,texSize), dtype=np.uint8)    
leftMask[:, :texSize//2+bandRadius] = 0

#Try creating luminance_alphamask
masked_texture = np.dstack((sinTex, leftMask)) #.reshape(sinTex.shape[0],-1)
masked_texture.shape

class MyApp(ShowBase):
    def __init__(self):
        ShowBase.__init__(self)

        #CREATE TEXTURE 
        self.sinTexture = Texture('masked_sin')
        self.sinTexture.setup2dTexture(texSize, texSize, Texture.T_unsigned_byte, Texture.F_luminance_alphamask) 
        self.sinTexture.setRamImage(masked_texture)   
                                                                      
        #CREATE SCENEGRAPH
        cm = CardMaker('card1')
        cm.setFrameFullscreenQuad()   
        self.card1 = self.aspect2d.attachNewNode(cm.generate())
        self.card1.setTransparency(TransparencyAttrib.MAlpha)  #enable transparency
        
        #SET TEXTURE STAGES
        self.card1.setTexture(self.sinTexture)   
        
if __name__ == '__main__':
    app = MyApp()
    app.run()