contract's guard validator with abstract type

I've this validation error : "Cannot compare two AssetTypes FunctionalAsset and Data that do not share a concrete supertype"

for this contract :

static contract functionalRelationInputAndOutputNotData(functionalRelation : FunctionalRelation, data1 : Data, data2 : Data) {
	severity = ERROR
	guard = functionalRelation.input != data1 || functionalRelation.output != data2
}

The corresponding meta model subset:

    AssetType Process extends (FunctionalAsset) {
        tags (Functional)
        attribute processType : EProcessType
        reference activities : Activity[*]
        reference supportingApplications : Application[*]
        reference involvedStakeholders : Stakeholder[*]
        reference involvedInformationSystems : InformationSystem[*]
        reference subProcess : Process[*]
    }

    AssetType Data extends (FunctionalAsset) {
        tags (Functional)
        attribute sensitive : Boolean
        reference storage : Storage[0..1]
    }

    AssetType FunctionalRelation {
        tags (Functional, Relation)
        reference input : FunctionalAsset[1]
        reference output : FunctionalAsset[1]
    }