type error in connection with labeling

Ladies and Gentlemen,

as part of my dissertation thesis, I am currently working on a job 
scheduling problem where I am at least exclusively using lists and labeling 
in a minimization. In this prolem, I am using an 'on 
execution'-status-variable which depends on a preparation factor (floating 
point) from the process-time.

Actually I could not find a way for the program to be processed. If I am 
just using the factor, I am getting an instatiation error. Using 
fix(floor(.)) gives a type error. What kan I do?

solve(...) :-
	length(Ruestzeit,AM),
	Ruestzeit :: 0..PLH,
	...
	ruestzei1(ZOR,Ruestanzg,Ruestzeit,97),
	time(minimize(assign(...),C)).

assign(...) :-
	...
	labeling(Ruestzeit),
	... .

ruestzei1([],[],[],_).
ruestzei1([ZOR|ZORs],[RA|RAs],[RZ|RZs],IO) :-
	name(T,[IO]),
	dauer(T,D),
	qa(T,QA),
	lqr(QA,ZOR,LF),
	fr(T,FR),
	rz(FR,RZF),
	hg(HG),
	RZ #= (RA*D*RZF*LF)*HG,
	ION is IO + 1,
	ruestzei1(ZORs,RAs,RZs,ION).

Surprisingly, using these clauses without related clauses works, but when I 
try to use 'RZ' in other clauses, the errors mentioned qboove appear.

Thank you very much in advance for an advice.

Greetings
Arne Ruban
Frankfurt University