The invention relates to detecting non-presence of nodes in a document of semi-structured data and to creating a values index which properly reflects this non-presence. The document of semi-structured data, comprising multiple hierarchically organized nodes, is scanned for values (step 110). Subsequently, these values are indexed (step 120). As non-presence of at least one node is detected (step 140), this non-presence is documented by enhancing the values index (12) by a pseudo key (20) (step 150).