diff --git a/types/rdf-ext/lib/Dataset.d.ts b/types/rdf-ext/lib/Dataset.d.ts
index a20bdad5c7..23b5125523 100644
--- a/types/rdf-ext/lib/Dataset.d.ts
+++ b/types/rdf-ext/lib/Dataset.d.ts
@@ -1,14 +1,16 @@
-import { Dataset } from 'rdf-js';
+import { Dataset, DatasetCore, Stream, Quad } from 'rdf-js';
 import { PropType } from './_PropType';
 import QuadExt = require('./Quad');
+import { Readable } from 'stream';
 
 interface DatasetExt extends Dataset {
   readonly length: number;
   toJSON(): Array<ReturnType<PropType<QuadExt, 'toJSON'>>>;
   clone(): this;
   readonly includes: PropType<Dataset, 'has'>;
-  merge(other: Dataset): DatasetExt;
+  merge(other: Dataset | Quad[]): this;
   removeMatches(subject: any, predicate: any, object: any, graph: any): this;
+  toStream(): Stream<QuadExt> & Readable;
 }
 
 export = DatasetExt;
diff --git a/types/rdf-ext/rdf-ext-tests.ts b/types/rdf-ext/rdf-ext-tests.ts
index 11e6c59604..f05dfb1df5 100644
--- a/types/rdf-ext/rdf-ext-tests.ts
+++ b/types/rdf-ext/rdf-ext-tests.ts
@@ -1,5 +1,5 @@
 import rdf = require('rdf-ext');
-import { Literal, Quad, Dataset, NamedNode, Stream, Sink } from 'rdf-js';
+import { Literal, Quad, Dataset, NamedNode, Stream, Sink, DatasetCore } from 'rdf-js';
 import QuadExt = require('rdf-ext/lib/Quad');
 import DataFactoryExt = require('rdf-ext/lib/DataFactory');
 import DatasetExt = require('rdf-ext/lib/Dataset');
@@ -228,8 +228,14 @@ function dataset_empty(): boolean {
     return rdf.dataset().length === 0;
 }
 
-function dataset_merge(): Dataset {
-    return rdf.dataset().merge(rdf.dataset());
+function dataset_merge(): DatasetExt {
+    const other: Dataset = <any> {};
+    return rdf.dataset().merge(other);
+}
+
+function dataset_merge_arrau(): DatasetExt {
+    const other: Quad[] = <any> {};
+    return rdf.dataset().merge(other);
 }
 
 function dataset_clone(): Dataset {