diff --git a/src/ol/source/Vector.js b/src/ol/source/Vector.js
index aed90d7369..c367084f21 100644
--- a/src/ol/source/Vector.js
+++ b/src/ol/source/Vector.js
@@ -941,7 +941,15 @@ class VectorSource extends Source {
    * @return {boolean} Is empty.
    */
   isEmpty() {
-    return this.featuresRtree_.isEmpty() && isEmpty(this.nullGeometryFeatures_);
+    if (this.featuresRtree_) {
+      return (
+        this.featuresRtree_.isEmpty() && isEmpty(this.nullGeometryFeatures_)
+      );
+    }
+    if (this.featuresCollection_) {
+      return this.featuresCollection_.getLength() === 0;
+    }
+    return true;
   }
 
   /**
diff --git a/test/browser/spec/ol/source/vector.test.js b/test/browser/spec/ol/source/vector.test.js
index 1f992d0e31..94beac641f 100644
--- a/test/browser/spec/ol/source/vector.test.js
+++ b/test/browser/spec/ol/source/vector.test.js
@@ -50,6 +50,34 @@ describe('ol.source.Vector', function () {
       it('returns true', function () {
         expect(vectorSource.isEmpty()).to.be(true);
       });
+      it('returns true without spatial index', function () {
+        const source = new VectorSource({
+          useSpatialIndex: false,
+        });
+        expect(source.isEmpty()).to.be(true);
+      });
+      it('returns false with geometry', function () {
+        vectorSource.addFeature(new Feature(new Point([0, 0])));
+        expect(vectorSource.isEmpty()).to.be(false);
+      });
+      it('returns false without spatial index and geometry', function () {
+        const source = new VectorSource({
+          useSpatialIndex: false,
+        });
+        source.addFeature(new Feature(new Point([0, 0])));
+        expect(source.isEmpty()).to.be(false);
+      });
+      it('returns false with null geometry', function () {
+        vectorSource.addFeature(new Feature());
+        expect(vectorSource.isEmpty()).to.be(false);
+      });
+      it('returns false without spatial index and null geometry', function () {
+        const source = new VectorSource({
+          useSpatialIndex: false,
+        });
+        source.addFeature(new Feature());
+        expect(source.isEmpty()).to.be(false);
+      });
     });
 
     describe('#addFeature', function () {