ClassCastException after running rules for a while in BRMS

Issue

• I get ClassCastException after running rules for a while

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.math.BigDecimal
    at ConditionEvaluator96bc4b8b5b1e4ac8ade8d7dc9012e0c6.evaluate(Unknown Source)
    at org.drools.core.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:248)
    at org.drools.core.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:204)
    at org.drools.core.reteoo.AlphaNode.assertObject(AlphaNode.java:141)
    at org.drools.core.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:494)
    at org.drools.core.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:384)
    at org.drools.core.reteoo.ObjectTypeNode.propagateAssert(ObjectTypeNode.java:298)
    at org.drools.core.phreak.PropagationEntry$Insert.execute(PropagationEntry.java:93)
    at org.drools.reteoo.common.RetePropagationList.addEntry(RetePropagationList.java:36)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.addPropagation(StatefulKnowledgeSessionImpl.java:1989)
    at org.drools.reteoo.common.ReteWorkingMemory.addPropagation(ReteWorkingMemory.java:229)
    at org.drools.core.reteoo.EntryPointNode.assertObject(EntryPointNode.java:224)
    at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:277)
    at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:237)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:1465)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:1420)

I understand that my rule condition calls a method with an incompatible class argument. But I'd like to know why it happens only after running for a while.

when
  $model : Model($model.isZero($model.getIntegerValue()))
then
  ...

public class Model {
    private Integer integerValue;
    public Integer getIntegerValue() {
        return integerValue;
    }
    public void setIntegerValue(Integer integerValue) {
        this.integerValue = integerValue;
    }
    public static boolean isZero(BigDecimal value) {
        if (value.compareTo(BigDecimal.ZERO) == 0) {
            return true;
        }
        return false;
    }
}