I am getting "Maximum call stack size exceeded" in MongooseJS

I am trying to update an object in mongoose:

Category.findByIdAndUpdate(product.category, {
      $addToSet: {
        products: product
      }
    }, function(err) {
      if (!err) {
        req.flash('success', 'Successfully updated')
        return res.redirect('back')
      } else {
        console.log("Error var" + err);

        res.render('products/edit', {
          title: 'Edit Product',
          product: product,
          errors: err.errors
        })
      }

    })

And I' m getting following error:

./node_modules/mongoose/lib/utils.js:413
        throw err;
              ^
RangeError: Maximum call stack size exceeded

Where am I wrong?