diff --git a/src/endpoint.ts b/src/endpoint.ts
index 1f9165b..535cd0f 100644
--- a/src/endpoint.ts
+++ b/src/endpoint.ts
@@ -217,6 +217,8 @@ export class Endpoint {
   async _fetchStructure() {
     const fromClause = this._settings.namedGraph ? `FROM <${this._settings.namedGraph}>` : ''
     const query = `
+      PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
+
       SELECT DISTINCT ?cls ?property ?linktype ?datatype
       ${fromClause}
       WHERE {