WebGLRenderingContext's texImage2D and texSubImage2D question

Hello Mattdesl, I have a one question about WebGLRenderingContext's texImage2D and texSubImage2D function.
What I trying to create is something similar like loading image by tile by tile.

Here is my first function. It's working properly while it's first load. But whenever loading second time using same function but tiled background.
It failed. Look at the code below

// This is first load code which is working fine

```
gl.bindTexture(gl.TEXTURE_2D, handle)
gl.texImage2D( gl.TEXTURE_2D, 0, gl.RGBA, width, height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null)

gl.bindTexture(gl.TEXTURE_2D, handle)
gl.texSubImage2D(gl.TEXTURE_2D, 0, x, y, gl.RGBA, gl.UNSIGNED_BYTE, image)
```

// This is second time using same but one different coding section [image data from canvas]. But it's not working.

```
gl.bindTexture(gl.TEXTURE_2D, handle)
gl.texImage2D( gl.TEXTURE_2D, 0, gl.RGBA, width, height, 0, gl.RGBA, gl.UNSIGNED_BYTE, [image data from canvas])

gl.bindTexture(gl.TEXTURE_2D, handle)
gl.texSubImage2D(gl.TEXTURE_2D, 0, x, y, gl.RGBA, gl.UNSIGNED_BYTE, image)
```

Have you ever face the problem similar like me. Then please guide me to the light. Much appreciate it.

Start by comparing the two texImage2D and texSubImage2D snippets, focusing on the change from a null data argument to image data from canvas. Consult the WebGLRenderingContext texture-upload behavior and determine what causes the second tiled load to fail; done means identifying a reproducible correction or required missing context.

Written by the indexing model from the issue text.