diff --git a/src/type/definition.d.ts b/src/type/definition.d.ts
index 457e756dd4..decfda65a1 100644
--- a/src/type/definition.d.ts
+++ b/src/type/definition.d.ts
@@ -433,7 +433,7 @@ export function argsToArgsConfig(
 export interface GraphQLObjectTypeConfig<TSource, TContext> {
   name: string;
   description?: Maybe<string>;
-  interfaces?: Thunk<Maybe<Array<GraphQLInterfaceType>>>;
+  interfaces?: Thunk<Array<GraphQLInterfaceType>>;
   fields: Thunk<GraphQLFieldConfigMap<TSource, TContext>>;
   isTypeOf?: Maybe<GraphQLIsTypeOfFn<TSource, TContext>>;
   extensions?: Maybe<Readonly<GraphQLObjectTypeExtensions<TSource, TContext>>>;
@@ -635,7 +635,7 @@ export class GraphQLInterfaceType {
 export interface GraphQLInterfaceTypeConfig<TSource, TContext> {
   name: string;
   description?: Maybe<string>;
-  interfaces?: Thunk<Maybe<Array<GraphQLInterfaceType>>>;
+  interfaces?: Thunk<Array<GraphQLInterfaceType>>;
   fields: Thunk<GraphQLFieldConfigMap<TSource, TContext>>;
   /**
    * Optionally provide a custom type resolver function. If one is not provided,
diff --git a/src/type/definition.js b/src/type/definition.js
index 4a7afed3cd..347b1dca14 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -772,7 +772,7 @@ function defineInterfaces(
     | GraphQLInterfaceTypeConfig<mixed, mixed>,
   >,
 ): Array<GraphQLInterfaceType> {
-  const interfaces = resolveThunk(config.interfaces) ?? [];
+  const interfaces = resolveThunk(config.interfaces ?? []);
   devAssert(
     Array.isArray(interfaces),
     `${config.name} interfaces must be an Array or a function which returns an Array.`,
@@ -875,7 +875,7 @@ export function argsToArgsConfig(
 export type GraphQLObjectTypeConfig<TSource, TContext> = {|
   name: string,
   description?: ?string,
-  interfaces?: Thunk<?Array<GraphQLInterfaceType>>,
+  interfaces?: Thunk<Array<GraphQLInterfaceType>>,
   fields: Thunk<GraphQLFieldConfigMap<TSource, TContext>>,
   isTypeOf?: ?GraphQLIsTypeOfFn<TSource, TContext>,
   extensions?: ?ReadOnlyObjMapLike<mixed>,
@@ -1086,7 +1086,7 @@ export class GraphQLInterfaceType {
 export type GraphQLInterfaceTypeConfig<TSource, TContext> = {|
   name: string,
   description?: ?string,
-  interfaces?: Thunk<?Array<GraphQLInterfaceType>>,
+  interfaces?: Thunk<Array<GraphQLInterfaceType>>,
   fields: Thunk<GraphQLFieldConfigMap<TSource, TContext>>,
   /**
    * Optionally provide a custom type resolver function. If one is not provided,