diff --git a/lib/relation-definition.js b/lib/relation-definition.js
index 5fe8af3d..fea24b65 100644
--- a/lib/relation-definition.js
+++ b/lib/relation-definition.js
@@ -783,7 +783,7 @@ HasMany.prototype.findById = function (fkId, options, cb) {
       return cb(err);
     }
     // Check if the foreign key matches the primary key
-    if (inst[fk] && inst[fk].toString() === modelInstance[pk].toString()) {
+    if (inst[fk] != null && inst[fk].toString() === modelInstance[pk].toString()) {
       cb(null, inst);
     } else {
       err = new Error('Key mismatch: ' + modelFrom.modelName + '.' + pk
@@ -1483,7 +1483,7 @@ BelongsTo.prototype.related = function (condOrRefresh, options, cb) {
           return cb(null, null);
         }
         // Check if the foreign key matches the primary key
-        if (inst[pk] && modelInstance[fk]
+        if (inst[pk] != null && modelInstance[fk] != null
           && inst[pk].toString() === modelInstance[fk].toString()) {
           self.resetCache(inst);
           cb(null, inst);
@@ -1891,7 +1891,7 @@ HasOne.prototype.related = function (condOrRefresh, options, cb) {
           return cb(null, null);
         }
         // Check if the foreign key matches the primary key
-        if (inst[fk] && modelInstance[pk]
+        if (inst[fk] != null && modelInstance[pk] != null
           && inst[fk].toString() === modelInstance[pk].toString()) {
           self.resetCache(inst);
           cb(null, inst);
@@ -3005,7 +3005,10 @@ ReferencesMany.prototype.findById = function (fkId, options, cb) {
     }
 
     var currentIds = ids.map(function(id) { return id.toString(); });
-    var id = (inst[pk] || '').toString(); // mongodb
+    var id = '';
+    if (inst[pk] != null) {
+      id = inst[pk].toString();
+    }
 
     // Check if the foreign key is amongst the ids
     if (currentIds.indexOf(id) > -1) {