Isis validation error because of optional property on Isis 1.8.0

[Erik de Hair]

While starting our application I get the following error:
nl.pocos.dom.billing.specification.AbstractTelephonySpecification#cdrExportId: 
incompatible Isis' default of required/optional properties vs JDO; add 
@javax.jdo.annotations.Column(allowNulls="...")

The property is defined as follows:

@Property(hidden=Where.EVERYWHERE)
    @Column(name = "export_id", allowsNull = "true")
    public int getCdrExportId()
    {
        return cdrExportId;
    }

Another property, defined likewise, has no problems:

@Column(name = "verlengingstermijn", allowsNull = "true")
    public int getAutomaticProlongationPeriod()
    {
        return automaticProlongationPeriod;
    }

I tried adding Optionalitity with no effect: 
@Property(hidden=Where.EVERYWHERE, optionality=Optionality.DEFAULT) and 
Optionality.OPTIONAL

I must be missing something.

Erik