diff --git a/src/LaunchDarkly/LDUser.php b/src/LaunchDarkly/LDUser.php
index fe8c14fa1..fb6ea96cb 100644
--- a/src/LaunchDarkly/LDUser.php
+++ b/src/LaunchDarkly/LDUser.php
@@ -117,7 +117,7 @@ public function toJSON() {
         if (isset($this->_lastName)) {
             $json['lastName'] = $this->_lastName;
         }
-        if (isset($this->_custom)) {
+        if (isset($this->_custom) && !empty($this->_custom)) {
             $json['custom'] = $this->_custom;
         }
         if (isset($this->_anonymous)) {
diff --git a/tests/LDUserTest.php b/tests/LDUserTest.php
index ca684f241..106b2a441 100644
--- a/tests/LDUserTest.php
+++ b/tests/LDUserTest.php
@@ -21,6 +21,13 @@ public function testCoerceLDUserKey() {
         $this->assertEquals("string", gettype($json['key']));
     }
 
+    public function testEmptyCustom() {        
+        $builder = new LDUserBuilder("foo@bar.com");
+        $user = $builder->build();
+        $json = $user->toJSON();
+        $this->assertTrue(!isset($json['custom']));
+    }
+
     public function testLDUserSecondary() {
         $builder = new LDUserBuilder("foo@bar.com");
         $user = $builder->secondary("secondary")->build();