It seems like $this->opaqueData can also be of type null; however, parameter $data of CommerceGuys\AuthNet\Req...nterface::addDataType() does only seem to accept CommerceGuys\AuthNet\DataTypes\DataTypeInterface, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation