diff --git a/types/openlayers/index.d.ts b/types/openlayers/index.d.ts
index 577e2f485a..42587298f0 100644
--- a/types/openlayers/index.d.ts
+++ b/types/openlayers/index.d.ts
@@ -11154,7 +11154,7 @@ declare module ol {
      *     Array.<Array.<ol.Coordinate>>), ol.geom.SimpleGeometry=):
      *     ol.geom.SimpleGeometry}
      */
-    type DrawGeometryFunctionType = (coords: (ol.Coordinate | ol.Coordinate[] | ol.Coordinate[][]), geo: ol.geom.SimpleGeometry) => ol.geom.SimpleGeometry;
+    type DrawGeometryFunctionType = (coords: (ol.Coordinate | ol.Coordinate[] | ol.Coordinate[][]), geo?: ol.geom.SimpleGeometry) => ol.geom.SimpleGeometry;
 
     /**
      * A function that takes an {@link ol.MapBrowserEvent} and returns a
diff --git a/types/openlayers/openlayers-tests.ts b/types/openlayers/openlayers-tests.ts
index de12feeb80..f0abcf28ce 100644
--- a/types/openlayers/openlayers-tests.ts
+++ b/types/openlayers/openlayers-tests.ts
@@ -19,6 +19,8 @@ let featureStyleFunction: ol.FeatureStyleFunction;
 let featureLoader: ol.FeatureLoader;
 let easingFunction: (t: number) => number;
 let drawGeometryFunction: ol.DrawGeometryFunctionType;
+drawGeometryFunction([0,0], new ol.geom.Point([0,0]));
+drawGeometryFunction([0,0]);
 
 // Type variables for OpenLayers
 let attribution: ol.Attribution;