diff --git a/lib/less/functions/types.js b/lib/less/functions/types.js
index 7502acf8..f6bfae92 100644
--- a/lib/less/functions/types.js
+++ b/lib/less/functions/types.js
@@ -57,24 +57,6 @@ functionRegistry.addMultiple({
     isem: function (n) {
         return isunit(n, 'em');
     },
-    isrem: function (n) {
-        return isunit(n, 'rem');
-    },
-    isvw: function (n) {
-        return isunit(n, 'vw');
-    },
-    isvh: function (n) {
-        return isunit(n, 'vh');
-    },
-    isvmin: function (n) {
-        return isunit(n, 'vmin');
-    },
-    isvmax: function (n) {
-        return isunit(n, 'vmax');
-    },
-    isch: function (n) {
-        return isunit(n, 'ch');
-    },
     isunit: isunit,
     unit: function (val, unit) {
         if (!(val instanceof Dimension)) {
diff --git a/lib/less/tree/unit.js b/lib/less/tree/unit.js
index 2aa0e8b8..755cf8a5 100644
--- a/lib/less/tree/unit.js
+++ b/lib/less/tree/unit.js
@@ -42,7 +42,7 @@ Unit.prototype.is = function (unitString) {
     return this.toString().toUpperCase() === unitString.toUpperCase();
 };
 Unit.prototype.isLength = function () {
-    return RegExp('^(px|em|rem|in|cm|mm|pc|pt|ex|vw|vh|vmin|vmax)$', 'g').test(this.toCSS());
+    return RegExp('^(px|em|rem|in|cm|mm|pc|pt|ex|vw|vh|vmin|vmax)$', 'gi').test(this.toCSS());
 };
 Unit.prototype.isEmpty = function () {
     return this.numerator.length === 0 && this.denominator.length === 0;
diff --git a/test/less/functions.less b/test/less/functions.less
index 431c2651..a95310a6 100644
--- a/test/less/functions.less
+++ b/test/less/functions.less
@@ -163,12 +163,12 @@
     pixel: ispixel(32px);
     percent: ispercentage(32%);
     em: isem(32em);
-    rem: isrem(32rem);
-    vw: isvw(32vw);
-    vh: isvh(32vh);
-    vmin: isvmin(32vmin);
-    vmax: isvmax(32vmax);
-    ch: isch(32ch);
+    rem: isunit(32rem, rem);
+    vw: isunit(32vw, vw);
+    vh: isunit(32vh, vh);
+    vmin: isunit(32vmin, vmin);
+    vmax: isunit(32vmax, vmax);
+    ch: isunit(32ch, ch);
     cat: isunit(32cat, cat);
     no-unit-is-empty: isunit(32, '');
     case-insensitive-1: isunit(32CAT, cat);