How to compile this template with short type?

[mrd]

[code]
Unsigned!T encodeZigZag( T )( inout T v ) pure
if( isSigned!( T ) )
{
     return v > 0
         ?
             v * 2
         :
             -v * 2 - 1;
}
unittest
{
     assert( encodeZigZag!long( 2147483647 ) == 4294967294 );
     assert( encodeZigZag!long( -2147483648 ) == 4294967295 );

     assert( encodeZigZag!short( 20 ) == 40 );
     assert( encodeZigZag!short( -20 ) == 39 );
}
[/code]

Error: cannot implicitly convert expression (cast(int)v > 0 ?
cast(int)v * 2 : cast(int)-v * 2 - 1) of type int to ushort

int?!