EvaluationException in Java

Hello.

I am trying to execute:

" PLI.sEvaluate("(defrelation mismatched-competitor ((?c1 company) (?c2 
company)) :<=> (and (competitor ?c1 c2) (> (absolute-value (- 
(number-of-employees ?c1) (number-of-employees ?c2))) 1000)))", 
"testModule", null); "

in Java, where "testModule" is the name of the module I am working in.

I get the following exception when I try to execute the above:

"edu.isi.stella.EvaluationException: While evaluating '(defrelation 
mismatched-competitor ((?c1 company) (?c2 company)) :<=> (and 
(competitor ?c1 c2) (> (absolute-value (- (number-of-employees ?c1) 
(number-of-employees ?c2))) |L|1000)))':
Undefined operator: `defrelation'
     at edu.isi.stella.Cons.evaluateConsTree(Cons.java:8180)
     at edu.isi.stella.Stella_Object.evaluate(Stella_Object.java:4341)
     at edu.isi.powerloom.PLI.evaluate(PLI.java:4373)
     at edu.isi.powerloom.PLI.sEvaluate(PLI.java:4416)".

Any ideas on why this is happening?

Thanks for your help.
Lee