diff --git a/lib/BlockingDriver.php b/lib/BlockingDriver.php
index 67e18bc..192127a 100644
--- a/lib/BlockingDriver.php
+++ b/lib/BlockingDriver.php
@@ -203,7 +203,7 @@ class BlockingDriver implements Driver {
     /**
      * {@inheritdoc}
      */
-    public function mkdir($path, $mode = 0644, $recursive) {
+    public function mkdir($path, $mode = 0644, $recursive = false) {
         return new Success((bool) @\mkdir($path, $mode, $recursive));
     }
 
diff --git a/lib/Driver.php b/lib/Driver.php
index 03b03e6..059ccf9 100644
--- a/lib/Driver.php
+++ b/lib/Driver.php
@@ -132,7 +132,7 @@ interface Driver {
      * @param bool $recursive
      * @return \Amp\Promise
      */
-    public function mkdir($path, $mode = 0644, $recursive);
+    public function mkdir($path, $mode = 0644, $recursive = false);
 
     /**
      * Delete a directory
diff --git a/lib/EioDriver.php b/lib/EioDriver.php
index b649ead..7be5555 100644
--- a/lib/EioDriver.php
+++ b/lib/EioDriver.php
@@ -356,7 +356,7 @@ class EioDriver implements Driver {
     /**
      * {@inheritdoc}
      */
-    public function mkdir($path, $mode = 0644, $recursive) {
+    public function mkdir($path, $mode = 0644, $recursive = false) {
         \call_user_func($this->incrementor, 1);
         $promisor = new Deferred;
         $priority = \EIO_PRI_DEFAULT;
diff --git a/lib/UvDriver.php b/lib/UvDriver.php
index a5c615a..68e498e 100644
--- a/lib/UvDriver.php
+++ b/lib/UvDriver.php
@@ -300,7 +300,7 @@ class UvDriver implements Driver {
     /**
      * {@inheritdoc}
      */
-    public function mkdir($path, $mode = 0644, $recursive) {
+    public function mkdir($path, $mode = 0644, $recursive = false) {
         $this->reactor->addRef();
         $promisor = new Deferred;