Delegate.CreateDelegate(Type, object, MethodInfo) throws ArgumentException "Cannot bind to the target method..."

I use a serializer "NetStack.BitBuffer". With reflection I get all read methods like Int32 ReadInt, UInt32 ReadUInt etc.

To speed up the invoke from the methodinfo I create delegates but suddently after next time I wanted to work and even was recording it to make vlog but same code shows exception. I debugged it and all values are correct but still it is not working.

public class BitBuffer // Recreated to test it
{
    public int ReadInt()
    {
        return 1;
    }
}

[Test]
public void DelegateCreateTest()
{
    BitBuffer bitBuffer = new BitBuffer();

    MethodInfo readIntMethod = bitBuffer.GetType().GetMethod("ReadInt");

    Assert.IsNotNull(readIntMethod, "Should not be null");

    var func = (Func<object>)Delegate.CreateDelegate(typeof(Func<object>), bitBuffer, readIntMethod);

    Assert.IsNotNull(func, "Should not be null");
}

The test code above worked before the exception started to show.

System.ArgumentException: 'Cannot bind to the target method because its signature is not compatible with that of the delegate type.'

If change my self to Func<int> then it works but you could guess it.. all other methods with other return type will not work.

If I google it actually shouldn't even possible to do this except post: The identical code which I found was here and seems like it should actually work: https://stackoverflow.com/a/44595323/5103256

I really get crazy like error in the matrix and cannot explain why suddently it is not working. Tooooo bad I didn't use git to follow what is different.