diff --git a/packages/grpc-js-core/src/call-credentials.ts b/packages/grpc-js-core/src/call-credentials.ts
index f0ad0980..888e4c62 100644
--- a/packages/grpc-js-core/src/call-credentials.ts
+++ b/packages/grpc-js-core/src/call-credentials.ts
@@ -2,33 +2,35 @@ import {map, reduce} from 'lodash';
 
 import {Metadata} from './metadata';
 
-export type CallMetadataGenerator<T extends {} = {}> =
-    (options: T, cb: (err: Error|null, metadata?: Metadata) => void) =>
+export type CallMetadataOptions = { service_url?: string; };
+
+export type CallMetadataGenerator =
+    (options: CallMetadataOptions, cb: (err: Error|null, metadata?: Metadata) => void) =>
         void;
 
 /**
  * A class that represents a generic method of adding authentication-related
  * metadata on a per-request basis.
  */
-export interface CallCredentials<T extends {} = {}> {
+export interface CallCredentials {
   /**
    * Asynchronously generates a new Metadata object.
    * @param options Options used in generating the Metadata object.
    */
-  generateMetadata(options: T): Promise<Metadata>;
+  generateMetadata(options: CallMetadataOptions): Promise<Metadata>;
   /**
    * Creates a new CallCredentials object from properties of both this and
    * another CallCredentials object. This object's metadata generator will be
    * called first.
    * @param callCredentials The other CallCredentials object.
    */
-  compose<S>(callCredentials: CallCredentials<S>): CallCredentials<S&T>;
+  compose(callCredentials: CallCredentials): CallCredentials;
 }
 
-class ComposedCallCredentials<T> implements CallCredentials<T> {
-  constructor(private creds: CallCredentials<T>[]) {}
+class ComposedCallCredentials implements CallCredentials {
+  constructor(private creds: CallCredentials[]) {}
 
-  async generateMetadata(options: T): Promise<Metadata> {
+  async generateMetadata(options: CallMetadataOptions): Promise<Metadata> {
     let base: Metadata = new Metadata();
     let generated: Metadata[] = await Promise.all(
         map(this.creds, (cred) => cred.generateMetadata(options)));
@@ -38,15 +40,15 @@ class ComposedCallCredentials<T> implements CallCredentials<T> {
     return base;
   }
 
-  compose<S>(other: CallCredentials<S>): CallCredentials<S&T> {
-    return new ComposedCallCredentials<S&T>([...this.creds, other]);
+  compose(other: CallCredentials): CallCredentials {
+    return new ComposedCallCredentials(this.creds.concat([other]));
   }
 }
 
-class SingleCallCredentials<T> implements CallCredentials<T> {
-  constructor(private metadataGenerator: CallMetadataGenerator<T>) {}
+class SingleCallCredentials implements CallCredentials {
+  constructor(private metadataGenerator: CallMetadataGenerator) {}
 
-  generateMetadata(options: T): Promise<Metadata> {
+  generateMetadata(options: CallMetadataOptions): Promise<Metadata> {
     return new Promise<Metadata>((resolve, reject) => {
       this.metadataGenerator(options, (err, metadata) => {
         if (metadata !== undefined) {
@@ -58,17 +60,17 @@ class SingleCallCredentials<T> implements CallCredentials<T> {
     });
   }
 
-  compose<S>(other: CallCredentials<S>): CallCredentials<S&T> {
-    return new ComposedCallCredentials<S&T>([this, other]);
+  compose(other: CallCredentials): CallCredentials {
+    return new ComposedCallCredentials([this, other]);
   }
 }
 
-class EmptyCallCredentials implements CallCredentials<{}> {
-  generateMetadata(options: {}): Promise<Metadata> {
+class EmptyCallCredentials implements CallCredentials {
+  generateMetadata(options: CallMetadataOptions): Promise<Metadata> {
     return Promise.resolve(new Metadata());
   }
 
-  compose<S>(other: CallCredentials<S>): CallCredentials<S> {
+  compose(other: CallCredentials): CallCredentials {
     return other;
   }
 }
@@ -81,12 +83,12 @@ export namespace CallCredentials {
    * generates a Metadata object based on these options, which is passed back
    * to the caller via a supplied (err, metadata) callback.
    */
-  export function createFromMetadataGenerator<T>(
-      metadataGenerator: CallMetadataGenerator<T>): CallCredentials<T> {
+  export function createFromMetadataGenerator(
+      metadataGenerator: CallMetadataGenerator): CallCredentials {
     return new SingleCallCredentials(metadataGenerator);
   }
 
-  export function createEmpty(): CallCredentials<{}> {
+  export function createEmpty(): CallCredentials {
     return new EmptyCallCredentials();
   }
 }
diff --git a/packages/grpc-js-core/src/channel-credentials.ts b/packages/grpc-js-core/src/channel-credentials.ts
index 98986ff4..419e3d17 100644
--- a/packages/grpc-js-core/src/channel-credentials.ts
+++ b/packages/grpc-js-core/src/channel-credentials.ts
@@ -7,19 +7,19 @@ import {CallCredentials} from './call-credentials';
  * as a set of per-call credentials, which are applied to every method call made
  * over a channel initialized with an instance of this class.
  */
-export interface ChannelCredentials<T extends {} = {}> {
+export interface ChannelCredentials {
   /**
    * Returns a copy of this object with the included set of per-call credentials
    * expanded to include callCredentials.
    * @param callCredentials A CallCredentials object to associate with this
    * instance.
    */
-  compose<S>(callCredentials: CallCredentials<S>): ChannelCredentials<S&T>;
+  compose(callCredentials: CallCredentials): ChannelCredentials;
 
   /**
    * Gets the set of per-call credentials associated with this instance.
    */
-  getCallCredentials(): CallCredentials<T>;
+  getCallCredentials(): CallCredentials;
 
   /**
    * Gets a SecureContext object generated from input parameters if this
@@ -29,28 +29,28 @@ export interface ChannelCredentials<T extends {} = {}> {
   getSecureContext(): SecureContext|null;
 }
 
-abstract class ChannelCredentialsImpl<T> implements ChannelCredentials<T> {
-  protected callCredentials: CallCredentials<T>;
+abstract class ChannelCredentialsImpl implements ChannelCredentials {
+  protected callCredentials: CallCredentials;
 
-  protected constructor(callCredentials?: CallCredentials<T>) {
+  protected constructor(callCredentials?: CallCredentials) {
     this.callCredentials = callCredentials || CallCredentials.createEmpty();
   }
 
-  abstract compose<S>(callCredentials: CallCredentials<S>): ChannelCredentialsImpl<S&T>;
+  abstract compose(callCredentials: CallCredentials): ChannelCredentialsImpl;
 
-  getCallCredentials(): CallCredentials<T> {
+  getCallCredentials(): CallCredentials {
     return this.callCredentials;
   }
 
   abstract getSecureContext(): SecureContext|null;
 }
 
-class InsecureChannelCredentialsImpl<T> extends ChannelCredentialsImpl<T> {
-  constructor(callCredentials?: CallCredentials<T>) {
+class InsecureChannelCredentialsImpl extends ChannelCredentialsImpl {
+  constructor(callCredentials?: CallCredentials) {
     super(callCredentials);
   }
 
-  compose<S>(callCredentials: CallCredentials<S>): ChannelCredentialsImpl<S&T> {
+  compose(callCredentials: CallCredentials): ChannelCredentialsImpl {
     throw new Error('Cannot compose insecure credentials');
   }
 
@@ -59,15 +59,15 @@ class InsecureChannelCredentialsImpl<T> extends ChannelCredentialsImpl<T> {
   }
 }
 
-class SecureChannelCredentialsImpl<T> extends ChannelCredentialsImpl<T> {
+class SecureChannelCredentialsImpl extends ChannelCredentialsImpl {
   secureContext: SecureContext;
 
-  constructor(secureContext: SecureContext, callCredentials?: CallCredentials<T>) {
+  constructor(secureContext: SecureContext, callCredentials?: CallCredentials) {
     super(callCredentials);
     this.secureContext = secureContext;
   }
 
-  compose<S>(callCredentials: CallCredentials<S>): ChannelCredentialsImpl<S&T> {
+  compose(callCredentials: CallCredentials): ChannelCredentialsImpl {
     const combinedCallCredentials =
         this.callCredentials.compose(callCredentials);
     return new SecureChannelCredentialsImpl(
@@ -86,6 +86,7 @@ function verifyIsBufferOrNull(obj: any, friendlyName: string): void {
 }
 
 export namespace ChannelCredentials {
+
   /**
    * Return a new ChannelCredentials instance with a given set of credentials.
    * The resulting instance can be used to construct a Channel that communicates
@@ -96,7 +97,7 @@ export namespace ChannelCredentials {
    */
   export function createSsl(
       rootCerts?: Buffer|null, privateKey?: Buffer|null,
-      certChain?: Buffer|null): ChannelCredentials<{}> {
+      certChain?: Buffer|null): ChannelCredentials {
     verifyIsBufferOrNull(rootCerts, 'Root certificate');
     verifyIsBufferOrNull(privateKey, 'Private key');
     verifyIsBufferOrNull(certChain, 'Certificate chain');
@@ -119,7 +120,7 @@ export namespace ChannelCredentials {
   /**
    * Return a new ChannelCredentials instance with no credentials.
    */
-  export function createInsecure(): ChannelCredentials<{}> {
+  export function createInsecure(): ChannelCredentials {
     return new InsecureChannelCredentialsImpl();
   }
 }
diff --git a/packages/grpc-js-core/src/index.ts b/packages/grpc-js-core/src/index.ts
index 7cd93562..4809647f 100644
--- a/packages/grpc-js-core/src/index.ts
+++ b/packages/grpc-js-core/src/index.ts
@@ -8,11 +8,9 @@ import { Metadata } from './metadata';
 import { IncomingHttpHeaders } from 'http';
 
 export interface OAuth2Client {
-  getRequestMetadata: (url: string) => Promise<{ Authorization: string }>;
+  getRequestMetadata: (url: string, callback: (err: Error|null, headers?: { Authorization: string }) => void) => void;
 }
 
-export type GoogleCallCredentials = CallCredentials<{ service_url: string }>;
-
 /**** Client Credentials ****/
 
 // Using assign only copies enumerable properties, which is what we want
@@ -22,18 +20,17 @@ export const credentials = Object.assign({
    * @param googleCredentials The authentication client to use.
    * @return The resulting CallCredentials object.
    */
-  createFromGoogleCredential: (googleCredentials: OAuth2Client): GoogleCallCredentials => {
-    return CallCredentials.createFromMetadataGenerator(async (options, callback) => {
-      let header;
-      try {
-        header = await googleCredentials.getRequestMetadata(options.service_url);
-      } catch (err) {
-        callback(err);
-        return;
-      }
-      const metadata = new Metadata();
-      metadata.add('authorization', header.Authorization);
-      callback(null, metadata);
+  createFromGoogleCredential: (googleCredentials: OAuth2Client): CallCredentials => {
+    return CallCredentials.createFromMetadataGenerator((options, callback) => {
+      googleCredentials.getRequestMetadata(options.service_url!, (err, headers) => {
+        if (err) {
+          callback(err);
+          return;
+        }
+        const metadata = new Metadata();
+        metadata.add('authorization', headers!.Authorization);
+        callback(null, metadata);
+      });
     });
   },
 
@@ -44,9 +41,9 @@ export const credentials = Object.assign({
    * @param callCredentials Any number of CallCredentials objects.
    * @return The resulting ChannelCredentials object.
    */
-  combineChannelCredentials: <S, T>(
-      channelCredentials: ChannelCredentials<S>,
-      ...callCredentials: CallCredentials<T>[]): ChannelCredentials<S&T> => {
+  combineChannelCredentials: (
+      channelCredentials: ChannelCredentials,
+      ...callCredentials: CallCredentials[]): ChannelCredentials => {
     return callCredentials.reduce((acc, other) => acc.compose(other), channelCredentials);
   },
 
@@ -56,9 +53,9 @@ export const credentials = Object.assign({
    * @param additional Any number of additional CallCredentials objects.
    * @return The resulting CallCredentials object.
    */
-  combineCallCredentials: <S, T>(
-      first: CallCredentials<S>,
-      ...additional: CallCredentials<T>[]): CallCredentials<S&T> => {
+  combineCallCredentials: (
+      first: CallCredentials,
+      ...additional: CallCredentials[]): CallCredentials => {
     return additional.reduce((acc, other) => acc.compose(other), first);
   }
 }, ChannelCredentials, CallCredentials);