GLSL 1.5 & OpenGL 3.3: Passing color to vertex shader seems to fail

I have a problem when passing vertex attributes to the running shader program. I'd like to pass two attributes, the position and a RGBA-color. Binding the attribute location works for the position. However, it does not work for the color. Therefore, all the geometry is eventually rendered black.

This is what I do:

GLuint programHandler;

// create program
// compile & attach vertex shader
// compile & attach fragment shader

glBindAttribLocation(programHandler, 0, "InPosition");
glBindAttribLocation(programHandler, 1, "InColor");
glBindFragDataLocation(programHandler, 1, "FragColor");

glLinkProgram(programHandler);
glUseProgram(programHandler);

// initialize uniform variables

// I'm trying to get the attribute locations now.
// I expect location "0" for InPosition and location "1" for InColor.
// In fact, it gets me a "-1" for InColor. I simply cannot see the reason for that behaviour

GLint positionLocation = glGetAttribLocation(programHandler, "InPosition"); // returns 0
GLint colorLocation = glGetAttribLocation(programHandler, "InColor"); // returns -1

glEnableVertexAttribArray(positionLocation);
glEnableVertexAttribArray(colorLocation); // fails!

My vertex shader is very basic. All I really do is transforming vertices and passing the color to the fragment shader:

#version 150 core

// input
uniform mat4 ModelviewMatrix;
uniform mat4 ProjectionMatrix;
in vec3 InPosition;
in vec4 InColor;

// output
out vec4 PassColor;

void main(void) {
    // passing color through to fragment shader
    PassColor = InColor;

    // transformation
    gl_Position = ProjectionMatrix * ModelviewMatrix * vec4(InPosition, 1.0);
}

My fragment shader should simply return that color:

#version 150 core
precision highp float;

// input
in vec4 PassColor;

// output
out vec4 FragColor;

void main(void) {
    FragColor = PassColor;
}

Why does binding "InPosition" work and "InColor" does not? I am aware that the GLSL compiler optimizes the shader code, so that unused variables cannot be bound. But, I don't see why the color should be optimized out here. After all, I use it by passing it to the fragment shader.