diff --git a/lib/less/parser.js b/lib/less/parser.js
index 68c0d603..c6c993e6 100644
--- a/lib/less/parser.js
+++ b/lib/less/parser.js
@@ -517,7 +517,7 @@ less.Parser = function Parser(env) {
                             // detect named color
                             return new(tree.Color)(tree.colors[k].slice(1));
                         } else {
-                            return new(tree.Keyword)(k) 
+                            return new(tree.Keyword)(k);
                         }
                     }
                 },
@@ -1159,16 +1159,20 @@ less.Parser = function Parser(env) {
                 }
             },
             condition: function () {
-                var a, b, op, index = i;
-                if ($('(')) {
-                    if (a = $(this.addition) || $(this.entities.keyword) || $(this.entities.quoted)) {
-                        if (op = $(/^(?:>=|=<|\!=|[<=>])/)) {
-                            if (b = $(this.addition) || $(this.entities.keyword) || $(this.entities.quoted)) {
-                                if ($(')')) {
-                                    return new(tree.Condition)(op, a, b, index);
-                                }
-                            }
+                var a, b, op, index = i, negate = false;
+                expect('(');
+                if ($('!')) { negate = true }
+                if (a = $(this.addition) || $(this.entities.keyword) || $(this.entities.quoted)) {
+                    if (op = $(/^(?:>=|=<|\!=|[<=>])/)) {
+                        if (b = $(this.addition) || $(this.entities.keyword) || $(this.entities.quoted)) {
+                            expect(')');
+                            return new(tree.Condition)(op, a, b, index);
+                        } else {
+                            error('expected expression');
                         }
+                    } else {
+                        expect(')');
+                        return new(tree.Condition)(negate ? '!=' : '=', a, new(tree.Keyword)('true'), index);
                     }
                 }
             },
diff --git a/test/css/mixins-guards.css b/test/css/mixins-guards.css
index aaeff5d4..fa20a160 100644
--- a/test/css/mixins-guards.css
+++ b/test/css/mixins-guards.css
@@ -30,3 +30,9 @@
 .default1 {
   content: default;
 }
+.test1 {
+  content: "true.";
+}
+.test2 {
+  content: "false.";
+}
diff --git a/test/less/mixins-guards.less b/test/less/mixins-guards.less
index d4640257..a99e14df 100644
--- a/test/less/mixins-guards.less
+++ b/test/less/mixins-guards.less
@@ -58,3 +58,14 @@
   content: default;
 }
 .default1 { .default }
+
+// true & false keywords
+.test (@a) when (@a) {
+    content: "true.";
+}
+.test (@a) when (!@a) {
+    content: "false.";
+}
+
+.test1 { .test(true) }
+.test2 { .test(false) }