OpenGL returns -1 for an uniform that does exist and is used

Question

I have this vertex shader:

#version 430 core

layout (location = 0) in vec3 position;
layout (location = 1) in vec2 textureCoordinate;
layout (location = 2) in vec3 normal;
layout (location = 4) in vec3 tangent;

uniform mat4 projectionMatrix;
uniform mat4 modelViewMatrix;
uniform vec3 lightPosition = vec3(0.0, 1.0, 0.0);

out vec2 texCoord;
out vec3 lightDirection;
out vec3 eyeDirection;

void main() {
    vec4 P = modelViewMatrix * vec4(position, 1.0);
    vec3 N = normalize(mat3(modelViewMatrix) * normal);
    vec3 T = normalize(mat3(modelViewMatrix) * tangent);
    vec3 B = cross(N, T);
    vec3 L = lightPosition - P.xyz;
    vec3 V = -P.xyz;

    lightDirection = normalize(vec3(dot(V, T), dot(V, B), dot(V, N)));
    eyeDirection = normalize(vec3(dot(V, T), dot(V, B), dot(V, N)));
    texCoord = textureCoordinate;

    gl_Position = projectionMatrix * P;
}

And this fragment shader:

#version 430 core

in vec2 texCoord;
in vec3 lightDirection;
in vec3 eyeDirection;

layout (binding = 0) uniform sampler2D tex;
layout (binding = 1) uniform sampler2D normalMap;

out vec4 color;

void main() {
    vec3 ambient = vec3(0.1);

    vec3 V = normalize(eyeDirection);
    vec3 L = normalize(lightDirection);
    vec3 N = normalize(texture(normalMap, texCoord)).rgb * 2.0 - vec3(1.0);
    vec3 R = reflect(-L, N);

    vec3 diffuseAlbedo = texture(tex, texCoord).rgb;
    /*vec3 diffuseAlbedo = vec3(1.0);*/
    vec3 diffuse = max(dot(N, L), 0.0) * diffuseAlbedo;

    vec3 specular = vec3(0.0);

    color = vec4(ambient + diffuse + specular, 1.0);
}

As you can see, the variable lightPosition is actually used in the calculations. But when I want to get the location of it via glGetUniformLocation, I get -1.

I looked through the other questions here on SO, like those ones:

Especially the last one - but in my case, lightPosition is used to calculate lightDirection and this is then used in the fragment shader. So it should not be removed.

Any ideas on what is going on here? Or how to debug this?

Also: I set a default value for the uniform. If I remove the default value, it still gives -1.

I am running Ubuntu 15.10 with Nvidia 340.96 drivers and a GF 710M card. I am running OpenGL 4.4 with core profile.

OpenGL returns -1 for an uniform that does exist and is used

1 Answers