mongoose markModified on array not working - node.js

Question

The following code returns this error: doc.validate is not a function
. I read that markModified should fix this...but it hasn't. What is wrong with this code?
StoreRequest.findOne({store: store._id}).populate('store').then(function(storeRequest) {
[0, 1, 2, 3, 4, 5].forEach(function(i) {
var newProductUrl = {stuff in here, including nested docs and arrays};
storeRequest.products[i] = newProductUrl;
storeRequest.markModified('products');
})
storeRequest.save().then(function(sr) {
return res.json({sr: sr});
})
}).catch(next)
Here is the StoreRequestSchema:
StoreRequest.js
var DataSchema = new mongoose.Schema({ ...
});
var ProductRequestSchema = new mongoose.Schema({
url: String,
product: { type: mongoose.Schema.Types.ObjectId, ref: 'Product'},
requests: [{type: mongoose.Schema.Types.ObjectId, ref: 'Request'}],
deal: { type: mongoose.Schema.Types.ObjectId, ref: 'Deal' },
data: DataSchema,
});
var ProductMatchSettingsSchema = new mongoose.Schema({
...
});
var SiteMapSchema = new mongoose.Schema({
...
});
var SpecialRequestSchema = new mongoose.Schema({
...
});
var StoreRequestSchema = new mongoose.Schema({
store: { type: mongoose.Schema.Types.ObjectId, ref: 'Store', unique: true},
products: [ProductRequestSchema],
siteMap: SiteMapSchema,
special: SpecialRequestSchema,
garbageUrls: [String] //Urls that have previously been thrown out
});