diff --git a/docs/change-log.md b/docs/change-log.md
index 3715279c..a6b93ab2 100644
--- a/docs/change-log.md
+++ b/docs/change-log.md
@@ -30,6 +30,7 @@
 - render: remove some redundancy within the component initialization code ([#2213](https://github.com/MithrilJS/mithril.js/pull/2213))
 - render: Align custom elements to work like normal elements, minus all the HTML-specific magic. ([#2221](https://github.com/MithrilJS/mithril.js/pull/2221))
 - render: simplify component removal ([#2214](https://github.com/MithrilJS/mithril.js/pull/2214))
+- cast className using toString ([#2309](https://github.com/MithrilJS/mithril.js/pull/2309))
 
 #### News
 
diff --git a/render/hyperscript.js b/render/hyperscript.js
index 36eabbc5..dce92e45 100644
--- a/render/hyperscript.js
+++ b/render/hyperscript.js
@@ -54,7 +54,7 @@ function execSelector(state, attrs, children) {
 	if (className != null || state.attrs.className != null) attrs.className =
 		className != null
 			? state.attrs.className != null
-				? state.attrs.className + " " + className
+				? String(state.attrs.className) + " " + String(className)
 				: className
 			: state.attrs.className != null
 				? state.attrs.className
diff --git a/render/tests/test-hyperscript.js b/render/tests/test-hyperscript.js
index bb4a0f62..c2b6c98d 100644
--- a/render/tests/test-hyperscript.js
+++ b/render/tests/test-hyperscript.js
@@ -358,6 +358,15 @@ o.spec("hyperscript", function() {
 
 			o(vnode.attrs.className).equals("a")
 		})
+		o("casts className using toString like browsers", function() {
+			const className = {
+				valueOf: () => ".valueOf",
+				toString: () => "toString"
+			}
+			var vnode = m("custom-element" + className, {className: className})
+
+			o(vnode.attrs.className).equals("valueOf toString")
+		})
 	})
 	o.spec("children", function() {
 		o("handles string single child", function() {