Android : Java Android OpenGL ES 2.0 Can't link program

I try to link a basic shader program with a vertex and fragment shader. Both shaders compile successfully, but if I try to link the program the linking always fails. Whats wrong, why the linking is failing? I run the MAIN code in the "onSurfaceCreated" method from the GLSurfaceView.Renderer interface.

MAIN:

String vertexShaderSource = "attribute vec4 a_Position;     " +
                            "void main()                    " +
                            "{                              " +
                            "   gl_Position = a_Position;   " +
                            "}                              ";

String fragmentShaderSource =   "precision mediump float;                   " +
                                "void main()                                " +
                                "{                                          " +
                                "   gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);" +
                                "}                                          ";

int vertexShaderHandle = GLES20.glCreateShader(GLES20.GL_VERTEX_SHADER);
GLES20.glShaderSource(vertexShaderHandle, vertexShaderSource);
ShaderHelper.compileShader(vertexShaderHandle);

int fragmentShaderHandle = GLES20.glCreateShader(GLES20.GL_FRAGMENT_SHADER);
GLES20.glShaderSource(fragmentShaderHandle, fragmentShaderSource);
ShaderHelper.compileShader(fragmentShaderHandle);

programHandle = GLES20.glCreateProgram();
GLES20.glAttachShader(programHandle, vertexShaderHandle);
GLES20.glAttachShader(programHandle, fragmentShaderHandle);
ProgramHelper.linkProgram(fragmentShaderHandle);

HELPER:

public static void compileShader(int shaderHandle)
    {
        GLES20.glCompileShader(shaderHandle);
        int[] results = new int[1];
        GLES20.glGetShaderiv(shaderHandle, GLES20.GL_COMPILE_STATUS, results, 0);

        if(results[0] != GLES20.GL_TRUE)
        {
            String infoLog = GLES20.glGetShaderInfoLog(shaderHandle);
            throw new RuntimeException("Failed to compile shader!" + '\n' + infoLog);
        }
    }

    public static void linkProgram(int programHandle)
    {
        GLES20.glLinkProgram(programHandle);
        int[] results = new int[1];
        GLES20.glGetProgramiv(programHandle, GLES20.GL_LINK_STATUS, results, 0);

        if(results[0] != GLES20.GL_TRUE)
        {
            String infoLog = GLES20.glGetProgramInfoLog(programHandle);
            throw new RuntimeException("Failed to link program!" + '\n' + infoLog);
        }
    }