diff --git a/lib/aggregate.js b/lib/aggregate.js
index a1ffb25ddfc..79296807ade 100644
--- a/lib/aggregate.js
+++ b/lib/aggregate.js
@@ -1100,13 +1100,13 @@ if (Symbol.asyncIterator != null) {
  */
 
 function isOperator(obj) {
-  if (typeof obj !== 'object') {
+  if (typeof obj !== 'object' || obj === null) {
     return false;
   }
 
   const k = Object.keys(obj);
 
-  return k.length === 1 && k.some(key => { return key[0] === '$'; });
+  return k.length === 1 && k[0][0] === '$';
 }
 
 /*!
diff --git a/lib/helpers/query/isOperator.js b/lib/helpers/query/isOperator.js
index 3b9813920ff..04488591a6c 100644
--- a/lib/helpers/query/isOperator.js
+++ b/lib/helpers/query/isOperator.js
@@ -7,5 +7,8 @@ const specialKeys = new Set([
 ]);
 
 module.exports = function isOperator(path) {
-  return path.startsWith('$') && !specialKeys.has(path);
-};
\ No newline at end of file
+  return (
+    path[0] === '$' &&
+    !specialKeys.has(path)
+  );
+};