diff --git a/src/util/defineClass.js b/src/util/defineClass.js
index c31488ced..157b1afc3 100644
--- a/src/util/defineClass.js
+++ b/src/util/defineClass.js
@@ -5,7 +5,7 @@ export default function(className, properties) {
   const decls = _.map(properties, (value, property) => {
     return postcss.decl({
       prop: _.kebabCase(property),
-      value: value,
+      value: `${value}`,
     })
   })