java.lang.IllegalStateException: Score corruption for 'employeeWeeklyHours' rule

I've been getting the error

Exception in thread "main" java.lang.IllegalStateException: Score corruption: the workingScore (-1300hard/0soft) is not the uncorruptedScore (-1320hard/0soft) after completedAction ([ShiftAssignment for shift: MONDAY, from 8:0 to 8:20, assigned employee: null => null, ShiftAssignment for shift: MONDAY, from 8:0 to 8:20, assigned employee: null => null]):
Score corruption analysis could not be generated because either corrupted constraintMatchEnabled (true) or uncorrupted constraintMatchEnabled (false) is disabled.
Check your score constraints manually.
at org.optaplanner.core.impl.score.director.AbstractScoreDirector.assertWorkingScoreFromScratch(AbstractScoreDirector.java:296)
at org.optaplanner.core.impl.phase.scope.AbstractPhaseScope.assertExpectedUndoMoveScore(AbstractPhaseScope.java:139)
at org.optaplanner.core.impl.constructionheuristic.decider.ConstructionHeuristicDecider.doMove(ConstructionHeuristicDecider.java:108)
at org.optaplanner.core.impl.constructionheuristic.decider.ConstructionHeuristicDecider.decideNextStep(ConstructionHeuristicDecider.java:77)
at org.optaplanner.core.impl.constructionheuristic.DefaultConstructionHeuristicPhase.solve(DefaultConstructionHeuristicPhase.java:67)
at org.optaplanner.core.impl.solver.DefaultSolver.runPhases(DefaultSolver.java:213)
at org.optaplanner.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:176)
at TeRo.TeRoAppMain.main(TeRoAppMain.java:25)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

when executing the following rule with an EasyScoreCalculator

// Employees are forced to work the exact amount specified in their contracts
rule "employeeWeeklyHours"
    when
		$employee : Employee( $weeklyHours : weeklyHours, weeklyHoursAreMandatory == true )
		$totalWorkAssignedMinutes : Number(intValue != $weeklyHours * 60)
			from accumulate( $shiftAssignment : ShiftAssignment( $shift : shift )
							 and eval($shiftAssignment.employeeIsAssigned($employee) == true)
			                 and Shift(this == $shift, $shiftTime : shiftTime),
							sum($shiftTime.getShiftTotalMinutes())
						   )
	then
		scoreHolder.addHardConstraintMatch( kcontext, -1 * Math.abs($totalWorkAssignedMinutes.intValue() - $weeklyHours * 60) );
end