diff --git a/realm/realm-library/src/objectServer/java/io/realm/mongodb/User.java b/realm/realm-library/src/objectServer/java/io/realm/mongodb/User.java
index c49de02135..738988ee63 100644
--- a/realm/realm-library/src/objectServer/java/io/realm/mongodb/User.java
+++ b/realm/realm-library/src/objectServer/java/io/realm/mongodb/User.java
@@ -114,6 +114,9 @@ protected PushImpl(OsPush osPush) {
     }
 
     User(OsSyncUser osUser, App app) {
+        if (osUser == null) {
+            throw new IllegalStateException("Non-null user required.");
+        }
         this.osUser = osUser;
         this.app = app;
         this.profile = new UserProfile(this);