diff --git a/lib/core/connection/connect.js b/lib/core/connection/connect.js
index bd10535466e..191b6c8446e 100644
--- a/lib/core/connection/connect.js
+++ b/lib/core/connection/connect.js
@@ -179,6 +179,9 @@ function prepareHandshakeDocument(authContext, callback) {
     }
 
     const authProvider = AUTH_PROVIDERS[credentials.mechanism];
+    if (authProvider == null) {
+      return callback(new MongoError(`No AuthProvider for ${credentials.mechanism} defined.`));
+    }
     authProvider.prepare(handshakeDoc, authContext, callback);
     return;
   }