diff --git a/lib/relation-definition.js b/lib/relation-definition.js
index 9279fc7e..490d27c2 100644
--- a/lib/relation-definition.js
+++ b/lib/relation-definition.js
@@ -467,6 +467,10 @@ HasMany.prototype.findById = function (fkId, cb) {
   filter.where[idName] = fkId;
   filter.where[fk] = modelInstance[pk];
   
+  if (filter.where[fk] === undefined) {
+    // Foreign key is undefined
+    return process.nextTick(cb);
+  }
   this.definition.applyScope(modelInstance, filter);
   
   modelTo.findOne(filter, function (err, inst) {
@@ -919,6 +923,11 @@ BelongsTo.prototype.related = function (refresh, params) {
       var query = {where: {}};
       query.where[pk] = modelInstance[fk];
       
+      if (query.where[pk] === undefined) {
+        // Foreign key is undefined
+        return process.nextTick(cb);
+      }
+    
       this.definition.applyScope(modelInstance, query);
       
       modelTo.findOne(query, function (err, inst) {