diff --git a/capi/geos_c.h.in b/capi/geos_c.h.in
index 271624334..37502912c 100644
--- a/capi/geos_c.h.in
+++ b/capi/geos_c.h.in
@@ -2580,6 +2580,8 @@ extern void GEOS_DLL *GEOSGeom_getUserData(const GEOSGeometry* g);
 * multi-geometry or collection or 1 for a simple geometry.
 * For nested collections, remember to check if returned
 * sub-geometries are **themselves** also collections.
+* Empty collection or multi-geometry types return 0,
+* and empty simple geometry types return 1.
 * \param g Input geometry
 * \return Number of direct children in this collection
 * \warning For GEOS < 3.2 this function may crash when fed simple geometries
diff --git a/include/geos/geom/Geometry.h b/include/geos/geom/Geometry.h
index 38343cee2..cfa989a9a 100644
--- a/include/geos/geom/Geometry.h
+++ b/include/geos/geom/Geometry.h
@@ -302,8 +302,13 @@ class GEOS_DLL Geometry {
     /// Return an integer representation of this Geometry type
     virtual GeometryTypeId getGeometryTypeId() const = 0; //Abstract
 
-    /// Returns the number of geometries in this collection
-    /// (or 1 if this is not a collection)
+    /**
+     * \brief Returns the number of geometries in this collection,
+     * or 1 if this is not a collection.
+     *
+     * Empty collection or multi-geometry types return 0,
+     * and empty simple geometry types return 1.
+     */
     virtual std::size_t
     getNumGeometries() const
     {