diff --git a/server/schema/schema.js b/server/schema/schema.js
index 15fa367..b8b8581 100644
--- a/server/schema/schema.js
+++ b/server/schema/schema.js
@@ -1,12 +1,12 @@
 const {
+  GraphQLSchema,
   GraphQLObjectType,
   GraphQLString,
   GraphQLID,
   GraphQLInt,
   GraphQLList,
-  GraphQLSchema
+  GraphQLNonNull
 } = require('graphql');
-const _ = require('lodash');
 
 const Book = require('../models/book');
 const Author = require('../models/author');
@@ -79,8 +79,8 @@ const Mutation = new GraphQLObjectType({
     addAuthor: {
       type: AuthorType,
       args: {
-        name: { type: GraphQLString },
-        age: { type: GraphQLInt }
+        name: { type: new GraphQLNonNull(GraphQLString) },
+        age: { type: new GraphQLNonNull(GraphQLInt) }
       },
       resolve (parent, { name, age }) {
         const author = new Author({
@@ -93,9 +93,9 @@ const Mutation = new GraphQLObjectType({
     addBook: {
       type: BookType,
       args: {
-        name: { type: GraphQLString },
-        genre: { type: GraphQLString },
-        authorId: { type: GraphQLID }
+        name: { type: new GraphQLNonNull(GraphQLString) },
+        genre: { type: new GraphQLNonNull(GraphQLString) },
+        authorId: { type: new GraphQLNonNull(GraphQLID) }
       },
       resolve (parent, { name, genre, authorId }) {
         const book = new Book({