diff --git a/src/util.js b/src/util.js
index 72733db..7e9d39a 100644
--- a/src/util.js
+++ b/src/util.js
@@ -9,6 +9,9 @@ const UPPERCASE_RE = /([A-Z])/g;
 const UPPERCASE_RE_TO_KEBAB = (match /* : string */)  /* : string */ => `-${match.toLowerCase()}`;
 
 export const kebabifyStyleName = (string /* : string */) /* : string */ => {
+    if (string[0] === '-' && string[1] === '-') {
+        return string;
+    }
     const result = string.replace(UPPERCASE_RE, UPPERCASE_RE_TO_KEBAB);
     if (result[0] === 'm' && result[1] === 's' && result[2] === '-') {
         return `-${result}`;
diff --git a/tests/util_test.js b/tests/util_test.js
index 1b2d774..7c61fc6 100644
--- a/tests/util_test.js
+++ b/tests/util_test.js
@@ -16,5 +16,8 @@ describe('Utils', () => {
         it('forces -ms-', () => {
             assert.equal(kebabifyStyleName('msFooBarBaz'), '-ms-foo-bar-baz');
         });
+        it('does not kebabify CSS Custom properties', () => {
+            assert.equal(kebabifyStyleName('--foo-Bar_Baz'), '--foo-Bar_Baz');
+        });
     });
 });