Contract class for a static class

Question

How to avoid the warining "Class 'XContract' is annotated as being the contract for the abstract class 'X' but doesn't extend the class"?
The class X static, static checker works ok with it, just the warining seems wrong:

[ContractClass(type(XContract))]
static class X
{
  [DllImport("some.dll")]
  public static extern void Y(int a);
}

[ContractClassFor(typeof(X))]
static class XContract
{
  public static void Y(int a)
  {
     Contract.Requires(a >= 0);
  }
}