diff --git a/lib/less/functions.js b/lib/less/functions.js
index e471e78bf..06361aa35 100644
--- a/lib/less/functions.js
+++ b/lib/less/functions.js
@@ -187,7 +187,7 @@ tree.functions = {
         if (typeof threshold === 'undefined') {
             threshold = 0.43;
         } else {
-            threshold = threshold.value;
+            threshold = number(threshold);
         }
         if (((0.2126 * (color.rgb[0]/255) + 0.7152 * (color.rgb[1]/255) + 0.0722 * (color.rgb[2]/255)) * color.alpha) < threshold) {
             return light;
diff --git a/test/css/functions.css b/test/css/functions.css
index 67db3cb6f..3268d7801 100644
--- a/test/css/functions.css
+++ b/test/css/functions.css
@@ -38,6 +38,10 @@
   contrast-dark-thresh: #eeeeee;
   contrast-high-thresh: #eeeeee;
   contrast-low-thresh: #111111;
+  contrast-light-thresh-per: #111111;
+  contrast-dark-thresh-per: #eeeeee;
+  contrast-high-thresh-per: #eeeeee;
+  contrast-low-thresh-per: #111111;
   format: "rgb(32, 128, 64)";
   format-string: "hello world";
   format-multiple: "hello earth 2";
diff --git a/test/less/functions.less b/test/less/functions.less
index 3b7cfd53a..9e509e9ef 100644
--- a/test/less/functions.less
+++ b/test/less/functions.less
@@ -42,6 +42,10 @@
   contrast-dark-thresh: contrast(#000, #111111, #eeeeee, 0.5);
   contrast-high-thresh: contrast(#555, #111111, #eeeeee, 0.6);
   contrast-low-thresh: contrast(#555, #111111, #eeeeee, 0.1);
+  contrast-light-thresh-per: contrast(#fff, #111111, #eeeeee, 50%);
+  contrast-dark-thresh-per: contrast(#000, #111111, #eeeeee, 50%);
+  contrast-high-thresh-per: contrast(#555, #111111, #eeeeee, 60%);
+  contrast-low-thresh-per: contrast(#555, #111111, #eeeeee, 10%);
   format: %("rgb(%d, %d, %d)", @r, 128, 64);
   format-string: %("hello %s", "world");
   format-multiple: %("hello %s %d", "earth", 2);