OpenGL bug?

README.cpp

//1. Create a shader program with the below frag and vertex shaders
//2. Call the below method passing "_projectionMat" as the uniformName (and the shaderPrograms ID)
//3. glGetUniformLocation() correctly identifies the location as 2
//4. glGetActiveUniform() incorrectly identifies the type as GL_SAMPLER_CUBE, instead of GL_FLOAT_MAT4
//5. Removing the explicit uniform locations from the vertex shader fixes this, therefore is layout(location for a uniform undefined behaviour, or is this a bug?

std::pair<int, GLenum> findUniform(const char *uniformName, int shaderProgram)
{
    int result = GL_CALL(glGetUniformLocation(shaderProgram, uniformName));
    if (result > -1)
    {
        GLenum type;
        GLint size;//Collect size, because its not documented that you can pass 0
        GL_CALL(glGetActiveUniform(shaderProgram, result, 0, 0, &size, &type, 0));
        return std::pair<int, GLenum>(result, type);
    }
    return  std::pair<int, GLenum>(-1, 0);
}

skybox.frag

#version 430

layout(location = 2) in vec3 tex_coords;

layout(location = 3) uniform samplerCube cube_texture;

out vec4 frag_colour;

void main () {
  frag_colour = texture(cube_texture, tex_coords);
}

skybox.vert

#version 430

layout(location = 0) in vec3 _vertex;
//layout(location = 1) in vec3 in_normal;
layout(location = 2) out vec3 tex_coords;

layout(location = 1) uniform mat4 _modelViewMat;
layout(location = 2) uniform mat4 _projectionMat;


void main () {
  tex_coords = _vertex;
  gl_Position = _projectionMat * _modelViewMat * vec4(_vertex, 1.0);
}