diff --git a/src/css/PropertyValuePart.js b/src/css/PropertyValuePart.js
index 55148d0a..cf909779 100644
--- a/src/css/PropertyValuePart.js
+++ b/src/css/PropertyValuePart.js
@@ -149,7 +149,7 @@ function PropertyValuePart(text, line, col){
     } else if (/^[\,\/]$/.test(text)){
         this.type   = "operator";
         this.value  = text;
-    } else if (/^[a-z\-\u0080-\uFFFF][a-z0-9\-\u0080-\uFFFF]*$/i.test(text)){
+    } else if (/^[a-z\-_\u0080-\uFFFF][a-z0-9\-_\u0080-\uFFFF]*$/i.test(text)){
         this.type   = "identifier";
         this.value  = text;
     }
diff --git a/tests/css/Validation.js b/tests/css/Validation.js
index 81e34ad4..1f051bef 100644
--- a/tests/css/Validation.js
+++ b/tests/css/Validation.js
@@ -69,7 +69,8 @@
             "foo",
             "foo, bar",
             "none, none",
-            "none, foo"
+            "none, foo",
+            "has_underscore"
         ],
 
         invalid: {