diff --git a/src/type/definition.d.ts b/src/type/definition.d.ts
index 32cf43d054..df62c66743 100644
--- a/src/type/definition.d.ts
+++ b/src/type/definition.d.ts
@@ -241,8 +241,10 @@ export function isNullableType(type: any): type is GraphQLNullableType;
 export function assertNullableType(type: any): GraphQLNullableType;
 
 export function getNullableType(type: void): undefined;
+export function getNullableType<T extends GraphQLNullableType>(type: T): T;
 export function getNullableType<T extends GraphQLNullableType>(
-  type: GraphQLNonNull<T> | T,
+  // eslint-disable-next-line @typescript-eslint/unified-signatures
+  type: GraphQLNonNull<T>,
 ): T;
 
 /**
diff --git a/src/type/definition.js b/src/type/definition.js
index 739732cd5f..2d5a15117d 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -474,7 +474,8 @@ export function assertNullableType(type: mixed): GraphQLNullableType {
 
 /* eslint-disable no-redeclare */
 declare function getNullableType(type: void | null): void;
-declare function getNullableType<T>(type: GraphQLNonNull<T> | T): T;
+declare function getNullableType<T: GraphQLNullableType>(type: T): T;
+declare function getNullableType<T>(type: GraphQLNonNull<T>): T;
 export function getNullableType(type) {
   /* eslint-enable no-redeclare */
   if (type) {