"Bl_shader, first string argument is not a valid uniform value". what

So I have an object called ‘Cube’ with this python code

import bge
scene = bge.logic.getCurrentScene()
objList = scene.objects
obj =  objList['Cube']

VertexShader = """
   uniform mat4 myMatrix;
   void main() // all vertex shaders define a main() function
      gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
         // this line transforms the predefined attribute gl_Vertex 
         // of type vec4 with the predefined uniform 
         // gl_ModelViewProjectionMatrix of type mat4 and stores 
         // the result in the predefined output variable gl_Position 
         // of type vec4. (gl_ModelViewProjectionMatrix combines 
         // the viewing transformation, modeling transformation and 
         // projection transformation in one matrix.)

FragmentShader = """
   uniform mat4 myMatrix;
   void main()
      gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
         // this fragment shader just sets the output color to opaque
         // red (red = 1.0, green = 0.0, blue = 0.0, alpha = 1.0)

mesh = obj.meshes[0]
mat = mesh.materials[0]
if hasattr( mat, 'getMaterialIndex') == True: 
    shader = mat.getShader()
    if shader != None:
        shader.setSource(VertexShader, FragmentShader, True)
        identMatrix = ([1,0,0,0], [0,1,0,0], [0,0,1,0], [0,0,0,1])
        shader.setUniformMatrix4("myMatrix", identMatrix, True)

But when I run this I got this error on shader.setUniforms’s line: Bl_shader, first string argument is not a valid uniform value
What am I doing wrong???

Found the solution, you have to use the matrix value to BOTH vertex and fragment shader, but for some reason the error log doesnt say that.

Edit: it seems that this only occures when you didn’t use the value made or using with Matrix value to the gl_FragColor