diff --git a/docs/change-log.md b/docs/change-log.md
index 1627c459..d889581a 100644
--- a/docs/change-log.md
+++ b/docs/change-log.md
@@ -18,6 +18,7 @@
 -	fix dom element ownership bug when mixing keyed elements and third party plugin elements [#463](https://github.com/lhorie/mithril.js/issues/463)
 -	fix edge case in flatten algorithm [#448](https://github.com/lhorie/mithril.js/issues/448)
 -	prevent unnecessary DOM move operation when mixing keyed and unkeyed elements [#398](https://github.com/lhorie/mithril.js/issues/398)
+-	revert [#382](https://github.com/lhorie/mithril.js/issues/382) due to diff regression [#512](https://github.com/lhorie/mithril.js/issues/512)
 
 ---
 
diff --git a/mithril.js b/mithril.js
index b9d2a974..8cd2181b 100644
--- a/mithril.js
+++ b/mithril.js
@@ -61,7 +61,8 @@ var m = (function app(window, undefined) {
 
 		for (var attrName in attrs) {
 			if (attrName === classAttrName) {
-				if (attrs[attrName] !== "") cell.attrs[attrName] = (cell.attrs[attrName] || "") + " " + attrs[attrName];
+				var className = cell.attrs[attrName]
+				cell.attrs[attrName] = (className && attrs[attrName] ? className + " " : className || "") + attrs[attrName];
 			}
 			else cell.attrs[attrName] = attrs[attrName]
 		}