diff --git a/lib/less/tree/value.js b/lib/less/tree/value.js
index 7f110f65..93630c3c 100644
--- a/lib/less/tree/value.js
+++ b/lib/less/tree/value.js
@@ -22,7 +22,7 @@ tree.Value.prototype = {
         for(i = 0; i < this.value.length; i++) {
             this.value[i].genCSS(env, output);
             if (i+1 < this.value.length) {
-                output.add(env.compress ? ',' : ', ');
+                output.add((env && env.compress) ? ',' : ', ');
             }
         }
     },
diff --git a/test/css/strings.css b/test/css/strings.css
index 522bae5e..cd6d6020 100644
--- a/test/css/strings.css
+++ b/test/css/strings.css
@@ -38,3 +38,6 @@
   color: #000000;
   color: #ffa500;
 }
+.watermark {
+  family: Univers, Arial, Verdana, San-Serif;
+}
diff --git a/test/less/strings.less b/test/less/strings.less
index 32fad721..c43e368d 100644
--- a/test/less/strings.less
+++ b/test/less/strings.less
@@ -49,3 +49,9 @@
     .mix-mul(black);
     .mix-mul(orange);
 }
+
+@test: Arial, Verdana, San-Serif;
+.watermark {
+  @family: ~"Univers, @{test}";
+  family: @family;
+}