diff --git a/lib/util.js b/lib/util.js
index 921a684d50c2fb..4f6d727eaa7391 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -113,6 +113,12 @@ exports.format = function(f) {
             str += f.slice(lastPos, i);
           str += '%';
           break;
+        default: // any other character is not a correct placeholder
+          if (lastPos < i)
+            str += f.slice(lastPos, i);
+          str += '%';
+          lastPos = i = i + 1;
+          continue;
       }
       lastPos = i = i + 2;
       continue;
diff --git a/test/parallel/test-util-format.js b/test/parallel/test-util-format.js
index 93998c598a0e1f..084e6f73b90ddf 100644
--- a/test/parallel/test-util-format.js
+++ b/test/parallel/test-util-format.js
@@ -126,6 +126,12 @@ assert.strictEqual(util.format('o: %j, a: %j', {}, []), 'o: {}, a: []');
 assert.strictEqual(util.format('o: %j, a: %j', {}), 'o: {}, a: %j');
 assert.strictEqual(util.format('o: %j, a: %j'), 'o: %j, a: %j');
 
+// Invalid format specifiers
+assert.strictEqual(util.format('a% b', 'x'), 'a% b x');
+assert.strictEqual(util.format('percent: %d%, fraction: %d', 10, 0.1),
+                   'percent: 10%, fraction: 0.1');
+assert.strictEqual(util.format('abc%', 1), 'abc% 1');
+
 {
   const o = {};
   o.o = o;