         }
 
         describe("optionalFrom<T: Convertible> -> [U : [T]]?") {
+            let field = ModelWithDictionaryProperty.Key.convertibleDictionaryProperty
+
             context("when the value cannot be casted to an NSDictionary") {
                 it("should return a nil") {
                     let value: NSArray = []
-                    let dict: NSDictionary = ["convertibleDictionaryProperty" : value]
+                    let dict: NSDictionary = [field.stringValue : value]
                     let mapper = KeyedMapper(JSON: dict, type: ModelWithDictionaryProperty.self)
-                    let field = ModelWithDictionaryProperty.Key.convertibleDictionaryProperty
+
                     let convertibleDictionary: [ConvertibleObject : [ConvertibleObject]]? = mapper.optionalFrom(field)
 
                     expect(convertibleDictionary).to(beNil())

                context("when one of the dictionary values cannot be cast to an NSArray") {
                    it("should return a typeMismatch error") {
                        let value: NSDictionary = ["" : [:]]
                        let dict: NSDictionary = ["convertibleDictionaryProperty" : value]
                        let mapper = KeyedMapper(JSON: dict, type: ModelWithDictionaryProperty.self)
                        let field = ModelWithDictionaryProperty.Key.convertibleDictionaryProperty
                        let convertibleDictionary: [ConvertibleObject : [ConvertibleObject]]? = mapper.optionalFrom(field)

                        expect(convertibleDictionary).to(beNil())

        }
        
        describe("optionalFrom<T: Convertible> -> [T]?") {
            let field = ModelWithArrayProperty.Key.convertibleArrayProperty

            context("when the value cannot be casted to an array of Any") {
                it("should return nil") {
                    let dict: NSDictionary = [field.stringValue : [:]]
                    let mapper = KeyedMapper(JSON: dict, type: ModelWithArrayProperty.self)
                    let convertibleArray: [ConvertibleObject]? = mapper.optionalFrom(field)
                    
                    expect(convertibleArray).to(beNil())
                }
