lookto tween time value not accepting variables

This could be a syntax or coding problem, but given the variable

<krpano foo="4"/>

the following command works as documented

lookto(0.1,0.2,0.12,tween("easeOutQuint",4));

but the following command appears to use a tween time of 0

lookto(0.1,0.2,0.12,tween("easeOutQuint",get(foo)));