diff --git a/lib/less/parser/parser.js b/lib/less/parser/parser.js
index 4bcfb8dc..3cec477a 100644
--- a/lib/less/parser/parser.js
+++ b/lib/less/parser/parser.js
@@ -503,7 +503,7 @@ var Parser = function Parser(context, imports, fileInfo) {
                         return;
                     }
 
-                    var value = parserInput.$re(/^([+-]?\d*\.?\d+)(%|[a-z]+)?/i);
+                    var value = parserInput.$re(/^([+-]?\d*\.?\d+)(%|[a-z_]+)?/i);
                     if (value) {
                         return new(tree.Dimension)(value[1], value[2]);
                     }