How can I upload pixels from a simple byte array to an OpenGl texture ?

I'm using glTexImage2D and all I get is a white rectangle instead of a pixelated texture. The 9th parameter (32-bit pointer to the pixel data) is IMO the problem. I tried lots of parameter types there (byte, ref byte, byte[], ref byte[], int & IntPtr + Marshall, out byte, out byte[], byte*). glGetError() always returns GL_NO_ERROR. There must be something I'm doing wrong because it's never some gibberish pixels. It's always white. glGenTextures works correct. The first id has the value 1 like always in OpenGL. And I draw colored lines without any problem. So something is wrong with my texturing. I'm in control of the DllImport. So I can change the parameter types if necessary.

GL.glBindTexture(GL.GL_TEXTURE_2D, id);
int w = 4;
int h = 4;
byte[] bytes = new byte[w * h * 4];
for (int i = 0; i < bytes.Length; i++)
    bytes[i] = (byte)Utils.random(256);
GL.glTexImage2D(GL.GL_TEXTURE_2D, 0, GL.GL_RGBA, w, h, 0, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, bytes);

[DllImport(GL_LIBRARY)] public static extern void glTexImage2D(uint what, int level, int internalFormat, int width, int height, int border, int format,
            int type, byte[] bytes);