Incompatible numeric controls that appear to be the same.

I have some system numeric controls, specifically "System: Numeric: System Spin Control", which is configured as a Double, 64Bit, ~15bit. In the context help window the 15bit is followed by [???]. When I try and connect the output of this control to a write double accessor in my class (or a double indicator) it results in a broken wire.

I haven't seen this before, guess, for one reason, I rarely use System controls.

Whaz Up? I can throw a type cast in, but that seems "ugly", would rather understand what is going on.

Thanks, as always.