Error in Action for missing values

dear all

i have one "Action" which retrieves 6 EntityReferences from opportunity entity, this works fine if all 6 EntityReferences have values on the said opportunity, but if one of those field is empty, then i receive the following error:

Entity Reference cannot have Id and Key Attributes empty. If you contact support, please provide the technical details.

knowing that all parameters in the action are not "Required" by definition.

thanks,.