diff --git a/src/model.js b/src/model.js
index 051d8d2bb3b6..eef43d15cb33 100644
--- a/src/model.js
+++ b/src/model.js
@@ -2979,7 +2979,7 @@ class Model {
                   if (include.association.through.model.rawAttributes[attr]._autoGenerated
                     || attr === include.association.foreignKey
                     || attr === include.association.otherKey
-                    || typeof associationInstance[include.association.through.model.name][attr] === undefined) {
+                    || typeof associationInstance[include.association.through.model.name][attr] === 'undefined') {
                     continue;
                   }
 
@@ -4300,7 +4300,7 @@ class Model {
                   if (include.association.through.model.rawAttributes[attr]._autoGenerated
                     || attr === include.association.foreignKey
                     || attr === include.association.otherKey
-                    || typeof instance[include.association.through.model.name][attr] === undefined) {
+                    || typeof instance[include.association.through.model.name][attr] === 'undefined') {
                     continue;
                   }