GraphQL with Typescript: Resolver Type Error

Using apollo-graphql, I've created an "account" type and accompanying resolver that uses graphql-iso-date. The configuration results in the following errors, any ideas on cause and resolution?

In the code included below, typedefs are combined into a collection from specific typedef modules.

Type '{ Date: GraphQLScalarType; DateTime: GraphQLScalarType; Time: GraphQLScalarType; } | { Query: AccountQueryResolver; }' is not assignable to type 'IResolvers | IResolvers[] | undefined'.

Type '{ Date: GraphQLScalarType; DateTime: GraphQLScalarType; Time: GraphQLScalarType; }' is not assignable to type 'IResolvers | IResolvers[] | undefined'.

Type '{ Date: GraphQLScalarType; DateTime: GraphQLScalarType; Time: GraphQLScalarType; }' is not assignable to type 'IResolvers'. Property 'Date' is incompatible with index signature

Type 'GraphQLScalarType' is not assignable to type '(() => any) | GraphQLScalarType | IEnumResolver | IResolverObject | IResolverOptions'.

Type 'GraphQLScalarType' is not assignable to type 'IResolverObject'.
Index signature is missing in type 'GraphQLScalarType'.

TypeDefs Collection

import CustomScalarTypeDefs from "./custom";
import QueryTypeDefs from "./query";
import AccountTypeDefs from "./account";

const TypeDefs = [CustomScalarTypeDefs, QueryTypeDefs, AccountTypeDefs];

export default TypeDefs;

Custom Scalar TypeDefs

import { gql } from "apollo-server-express";

const customScalarTypeDefs = gql`
  scalar Date
  scalar DateTime
  scalar Time
`;

export default customScalarTypeDefs;

Query TypeDefs

import { gql } from "apollo-server-express";

const QueryTypeDefs = gql`
  type Query {
    account(id: String!): Account
  }
`;

export default QueryTypeDefs;

Custom Scalar Resolver

import { GraphQLDate, GraphQLDateTime, GraphQLTime } from "graphql-iso-date";

const CustomScalarResolver = {
  Date: GraphQLDate,
  DateTime: GraphQLDateTime,
  Time: GraphQLTime
};

export default CustomScalarResolver;

Account Resolver

import { QueryById } from "./signatures";
import Account, { AccountModel, AccountPayload } from "../../models/account";

interface AccountQueryResolver {
  account: (_: never, req: QueryById) => Promise<AccountModel | null>;
  createAccount: (
    parent: Account | null,
    { entity, name }: AccountPayload
  ) => Promise<AccountModel | null>;
}

const accountResolver: AccountQueryResolver = {
  account: (_, { id }): Promise<AccountModel | null> => {
    return Account.findOne({ _id: id }).exec();
  },
  createAccount: (_, { entity, name }): Promise<AccountModel | null> => {
    return Account.create({ entity, name });
  }
};

export default accountResolver;