Prolog error arguments are not instantiated

When running this code:

signcheck(Sign1,Sign2):-(Sign1==clubs,Sign2==hearts);(Sign1==clubs,Sign2==spades)
                            ;(Sign1==clubs,Sign2==diamonds);(Sign1==hearts,Sign2==spades)
                            ;(Sign1==hearts,Sign2==diamonds);(Sign1==spades,Sign2==diamonds).
    lowercard(card(Num1,Sign1),card(Num2,_),card(Num1,Sign1)):- Num1<Num2.
    lowercard(card(Num1,Sign1),card(Num2,Sign2),card(Num1,Sign1)):- Num1==Num2,signcheck(Sign1,Sign2).
    lowercard(card(Num1,Sign1),card(Num2,Sign2),card(Num2,Sign2)):- Num1==Num2,signcheck(Sign2,Sign1).
    lowercard(card(Num1,_),card(Num2,Sign2),card(Num2,Sign2)):- Num1>Num2.
    lowest([card(X,Y)],card(X,Y)).
    lowest([X|XS],Z):- lowercard(X,Y,Z), lowest(XS,Y).

when running the query:

lowest([card(5, hearts),card(4,diamonds)], X)

I get this error message:

Arguments are not sufficiently instantiated
In:
   [3] 5<_1600
   [2] lowercard(card(5,hearts),card(_1668,_1670),card(5,hearts)) at  line 4
   [1] lowest([card(5,hearts),...],card(5,hearts)) at  line 8

Why is this occouring?

Solution

Arithmetic operations (such as <) cannot be evaluated unless all arguments are instantiated. If your case, the right-hand argument (Num2, it looks like) hasn't been instantiated.