[Python] what is this unnamed error and how to fix it?

I am working on a costume 3D game engine which uses Python and the ModernGL library. Every time I run the code, I get this error for my shader program:

Traceback (most recent call last):
File "h:\Projects\Python\HyperDoom project\main.py", line 20, in <module>
game: Game = Game("ROMS\\DOOM.WAD")
~~~~^^^^^^^^^^^^^^^^^^
File "h:\Projects\Python\HyperDoom project\main.py", line 11, in __init__
self.Engine: Engine = Engine((1280, 720), self.E1M1, self.wadData)
~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "h:\Projects\Python\HyperDoom project\engin3d.py", line 29, in __init__
self.lData: LevelData = LevelData(data, self)
~~~~~~~~~^^^^^^^^^^^^
File "h:\Projects\Python\HyperDoom project\level_data.py", line 26, in __init__
self.mesh: Mesh = Mesh(self.ctx, self)
~~~~^^^^^^^^^^^^^^^^
File "h:\Projects\Python\HyperDoom project\Mesh.py", line 8, in __init__
self.vaos: VAOs = VAOs(ctx, level)
~~~~^^^^^^^^^^^^
File "h:\Projects\Python\HyperDoom project\VAOs.py", line 13, in __init__
self.vaos["level/test"] = self.getVao(
~~~~~~~~~~~^
self.programs.programs['test'],
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
self.vbos.vbos['level']
^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "h:\Projects\Python\HyperDoom project\VAOs.py", line 23, in getVao
vao = self.ctx.vertex_array(program, [(vbo, '3f 3f 2f 1i', "in_position", "in_color", "in_uv", "in_layer")])
File "C:\Users\Yusof\AppData\Local\Programs\Python\Python313\Lib\site-packages\moderngl\__init__.py", line 1901, in vertex_array
return self._vertex_array(*args, **kwargs)
~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
File "C:\Users\Yusof\AppData\Local\Programs\Python\Python313\Lib\site-packages\moderngl\__init__.py", line 1918, in _vertex_array
attribs = [types[x] if type(x) is int else types[locations[x]] for x in attribs]
~~~~~~~~~^^^
KeyError: 'in_layer'


I don't know what this error means. It even doesn't have a name or explanation. It just writes the variable's name.

This is my vertex shader:

#version 330 core

layout (location = 0) in vec3 in_position;
layout (location = 1) in vec3 in_color;
layout (location = 2) in vec2 in_uv;
layout (location = 3) in int in_layer;

out vec4 frag_color;
out vec2 frag_uv;
flat out int frag_layer;

uniform mat4 m_proj;
uniform mat4 m_view;
uniform mat4 m_model;

void main() {
frag_color = vec4(in_color, 1.0);
frag_uv = in_uv;
frag_layer = in_layer;
gl_Position = m_proj * m_view * m_model * vec4(in_position, 1);
}


and this my fragment shader:

#version 330 core

in vec4 frag_color;
in vec2 frag_uv;

flat in int frag_layer;

out vec4 color;

uniform sampler2DArray flats;

void main() {
color = texture(flats, vec3(frag_uv, 0)) + (frag_color) * 2 - frag_color - frag_color + frag_layer * 2;
}


In case the problem is from vao, this the definition:

vao = self.ctx.vertex_array(program, [(vbo, '3f 3f 2f 1i', "in_position", "in_color", "in_uv", "in_layer")])


What is this error and how can I fix it?

At first, I thought this error was because that I didn't use that variable, which is why I wrote such ca omplex expression for frag_color. But it seems that this is not the case.

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