diff --git a/lib/schema.js b/lib/schema.js
index c8453fa4307..6464e6c8b52 100644
--- a/lib/schema.js
+++ b/lib/schema.js
@@ -503,7 +503,7 @@ Schema.prototype.defaultOptions = function(options) {
  */
 
 Schema.prototype.discriminator = function(name, schema) {
-  this._applyDiscriminators = Object.assign({}, this._applyDiscriminators, { [name]: schema });
+  this._applyDiscriminators = Object.assign(this._applyDiscriminators || {}, { [name]: schema });
 
   return this;
 };