Does operator fail to distinguish short[] from ushort[]?

Question

Consider the following code:

I build it with .NET framework 4.7.2.

ushort[] a = new ushort[100];

if (a is short[])
{
    Assert.Fail("This never happens.");
}
object b = a;

if (b is short[])
{
    Assert.Fail("Why?");
}

The first test works as expected (a is not of short[]). My question concerns the second test, which, unexpectedly, does not fail.

Why is it so?

EDIT: added MSIL (Release, optimized)

 .maxstack  8
  IL_0000:  ldc.i4.s   100
  IL_0002:  newarr     [mscorlib]System.UInt16
  IL_0007:  isinst     int16[]
  IL_000c:  brfalse.s  IL_0018
  IL_000e:  ldstr      "Why\?"
  IL_0013:  call       void [(...).TestFramework](...).Assert::Fail(string)
  IL_0018:  ret