diff --git a/slugify.js b/slugify.js
index f5a6a1e..0befa5e 100644
--- a/slugify.js
+++ b/slugify.js
@@ -36,6 +36,8 @@
         var appendChar = locale[ch];
         if (appendChar === undefined) appendChar = charMap[ch];
         if (appendChar === undefined) appendChar = ch;
+        // A null mapping (e.g. extend({ '%': null })) removes the character.
+        if (appendChar === null) appendChar = '';
         if (appendChar === replacement) appendChar = ' ';
         return result + appendChar
           // remove not allowed characters
diff --git a/test/slugify.js b/test/slugify.js
index e2581d2..0635c26 100644
--- a/test/slugify.js
+++ b/test/slugify.js
@@ -285,4 +285,16 @@ describe('slugify', () => {
     delete require.cache[require.resolve('../')]
     slugify = require('../')
   })
+
+  it('removes characters mapped to null via extend()', () => {
+    // https://github.com/simov/slugify/issues/177
+    slugify.extend({ '|': null, '%': null, $: null })
+    t.equal(slugify('100%'), '100')
+    t.equal(slugify('a|b$c'), 'abc')
+
+    delete require.cache[require.resolve('../')]
+    slugify = require('../')
+
+    t.equal(slugify('100%'), '100percent')
+  })
 })