Lookup fields and null state

Lookup fields and null state

Hi All,

I have a simple bit of code that doesn't appear to be playing by the rules! 

Both  Task_Helicopters_Required and  Task_Pilots_Required are non bidirectional Lookup fields.

if (Task_Helicopters_Required = null ) && (Task_Pilots_Required = null)
{
enable Task_Type;
}
else
{
disable Task_Type;
}

The above is accepted as valid script but Task_Type is immediately disabled on form load. 

I have tried changing null to "" and again it's accepted but in this case, Task_Type is never disabled.

Am I being stupid?