diff --git a/lib/less/parser.js b/lib/less/parser.js
index b1a70654..24afec0d 100644
--- a/lib/less/parser.js
+++ b/lib/less/parser.js
@@ -814,7 +814,7 @@ less.Parser = function Parser(env) {
 
                 if (! $('[')) return;
 
-                if (key = $(/^[a-z-]+/) || $(this.entities.quoted)) {
+                if (key = $(/^[a-zA-Z-]+/) || $(this.entities.quoted)) {
                     if ((op = $(/^[|~*$^]?=/)) &&
                         (val = $(this.entities.quoted) || $(/^[\w-]+/))) {
                         attr = [key, op, val.toCSS ? val.toCSS() : val].join('');