diff --git a/compat/operator/distinctUntilKeyChanged.ts b/compat/operator/distinctUntilKeyChanged.ts
index 59f0787cc89..655e3de7b43 100644
--- a/compat/operator/distinctUntilKeyChanged.ts
+++ b/compat/operator/distinctUntilKeyChanged.ts
@@ -3,8 +3,8 @@ import { Observable } from 'rxjs';
 import { distinctUntilKeyChanged as higherOrder } from 'rxjs/operators';
 
 /* tslint:disable:max-line-length */
-export function distinctUntilKeyChanged<T>(this: Observable<T>, key: string): Observable<T>;
-export function distinctUntilKeyChanged<T>(this: Observable<T>, key: string, compare: <K> (x: K, y: K) => boolean): Observable<T>;
+export function distinctUntilKeyChanged<T>(this: Observable<T>, key: keyof T): Observable<T>;
+export function distinctUntilKeyChanged<T>(this: Observable<T>, key: keyof T, compare: <K> (x: K, y: K) => boolean): Observable<T>;
 /* tslint:enable:max-line-length */
 
 /**
@@ -64,6 +64,6 @@ export function distinctUntilKeyChanged<T>(this: Observable<T>, key: string, com
  * @method distinctUntilKeyChanged
  * @owner Observable
  */
-export function distinctUntilKeyChanged<T>(this: Observable<T>, key: string, compare?: <K> (x: K, y: K) => boolean): Observable<T> {
+export function distinctUntilKeyChanged<T>(this: Observable<T>, key: keyof T, compare?: <K> (x: K, y: K) => boolean): Observable<T> {
   return higherOrder<T>(key, compare)(this);
 }
diff --git a/src/internal/operators/distinctUntilKeyChanged.ts b/src/internal/operators/distinctUntilKeyChanged.ts
index b9c6cc2fa63..3e0adb8a242 100644
--- a/src/internal/operators/distinctUntilKeyChanged.ts
+++ b/src/internal/operators/distinctUntilKeyChanged.ts
@@ -2,8 +2,8 @@ import { distinctUntilChanged } from './distinctUntilChanged';
 import { MonoTypeOperatorFunction } from '../types';
 
 /* tslint:disable:max-line-length */
-export function distinctUntilKeyChanged<T>(key: string): MonoTypeOperatorFunction<T>;
-export function distinctUntilKeyChanged<T>(key: string, compare: <K> (x: K, y: K) => boolean): MonoTypeOperatorFunction<T>;
+export function distinctUntilKeyChanged<T>(key: keyof T): MonoTypeOperatorFunction<T>;
+export function distinctUntilKeyChanged<T>(key: keyof T, compare: <K> (x: K, y: K) => boolean): MonoTypeOperatorFunction<T>;
 /* tslint:enable:max-line-length */
 
 /**
@@ -70,6 +70,6 @@ export function distinctUntilKeyChanged<T>(key: string, compare: <K> (x: K, y: K
  * @method distinctUntilKeyChanged
  * @owner Observable
  */
-export function distinctUntilKeyChanged<T>(key: string, compare?: <K> (x: K, y: K) => boolean): MonoTypeOperatorFunction<T> {
+export function distinctUntilKeyChanged<T>(key: keyof T, compare?: <K> (x: K, y: K) => boolean): MonoTypeOperatorFunction<T> {
   return distinctUntilChanged((x: T, y: T) => compare ? compare(x[key], y[key]) : x[key] === y[key]);
 }