diff --git a/src/reagent/impl/util.cljs b/src/reagent/impl/util.cljs
index 3f100c8..af28b93 100644
--- a/src/reagent/impl/util.cljs
+++ b/src/reagent/impl/util.cljs
@@ -147,8 +147,8 @@
            rst)))
 
 (defn- merge-class [p1 p2]
-  (if-let [names (class-names (:class p1) (:class p2))]
-    (assoc p2 :class names)
+  (if (or (contains? p1 :class) (contains? p2 :class))
+    (assoc p2 :class (class-names (:class p1) (:class p2)))
     p2))
 
 (defn- merge-style [p1 p2]