diff --git a/src/util.js b/src/util.js
index c3f90e36..6edd68de 100644
--- a/src/util.js
+++ b/src/util.js
@@ -142,6 +142,9 @@ module.exports = {
    * @return {String} A native javascript string
    */
   decode_utf8: function (utf8) {
+    if (typeof utf8 !== 'string') {
+      throw new Error('Parameter "utf8" is not of type string');
+    }
     try {
       return decodeURIComponent(escape(utf8));
     } catch (e) {
diff --git a/test/general/basic.js b/test/general/basic.js
index 26c140f0..7c090dea 100644
--- a/test/general/basic.js
+++ b/test/general/basic.js
@@ -328,4 +328,13 @@ describe('Basic', function() {
     });
   });
 
+  describe("Misc.", function() {
+
+    it('util.decode_utf8 throws error if invalid parameter type', function () {
+      var test = openpgp.util.decode_utf8.bind(null, {chameleon: true});
+      expect(test).to.throw(Error, /Parameter "utf8" is not of type string/);
+    });
+
+  });
+
 });