OpenGL - uniform presence causing shader to be bypassed - opengl

Question

UPDATE: So it turns out this was due to a bug in the C side of things, causing some of the matrix to become malformed. The shaders are all fine. So if adding uniforms causes weird things to happen, my advice would be to use a debugger to check the value of ALL uniforms and make sure that they are all being set correctly.
So I am trying to render depth to a cube map to use as a shadow map, but when I add and use a uniform in the fragment shader everything becomes white as if the shader isn't being used. No warnings or errors are generated when compiling/linking the shader.
The shader program I am using to render the depth map (setting the depth simply to the fragment z position as a test) is as follows:
//vertex shader
#version 430
in layout(location=0) vec4 vertexPositionModel;
uniform mat4 modelToWorldMatrix;
void main() {
gl_Position = modelToWorldMatrix * vertexPositionModel;
}
//geometry shader
#version 430
layout (triangles) in;
layout (triangle_strip, max_vertices=18) out;
out vec4 fragPositionWorld;
uniform mat4 projectionMatrices[6];
void main() {
for (int face = 0; face < 6; face++) {
gl_Layer = face;
for (int i = 0; i < 3; i++) {
fragPositionWorld = gl_in[i].gl_Position;
gl_Position = projectionMatrices[face] * fragPositionWorld;
EmitVertex();
}
EndPrimitive();
}
}
//Fragment shader
#version 430
in vec4 fragPositionWorld;
void main() {
gl_FragDepth = abs(fragPositionWorld.z);
}
The main shader samples from the cubemap and simply renders the depth as greyscale colour:
vec3 lightDirection = fragPositionWorld - pointLight.position;
float closestDepth = texture(shadowMap, lightDirection).r;
finalColour = vec4(vec3(closestDepth), 1.0);
The scene is a small cube in a larger cubic room, and renders as expected, dark near z = 0 and the cube projected back onto the wall (The depth map is being rendered from the centre of the room):
Good:
[2
I can move the small cube around and the projection projects correctly onto all the sides of the cubemap. All good so far.
The problem is when I add a uniform to the fragment shader, i.e:
#version 430
in vec4 fragPositionWorld;
uniform vec3 lightPos;
void main() {
gl_FragDepth = min(lightPos.y, 0.5);
}
Everything renders as white, same as if the render failed to compile:
Bad:
gDEBugger reports that the uniform is set correctly (0,4,0) but regardless of what that lightPos is, gl_FragDepth should be set to a value less than 0.5 and appear a shade of grey (which is what happens if I set gl_FragDepth = 0.5 directly), so I can only conclude that the fragment shader is not being used for some reason and the default one is being use instead. Unfortunately I have no idea why.