diff --git a/src/vs/workbench/contrib/webview/electron-browser/webviewElement.ts b/src/vs/workbench/contrib/webview/electron-browser/webviewElement.ts
index 00aa85761895c..e2a484ad05e99 100644
--- a/src/vs/workbench/contrib/webview/electron-browser/webviewElement.ts
+++ b/src/vs/workbench/contrib/webview/electron-browser/webviewElement.ts
@@ -113,24 +113,30 @@ class WebviewSession extends Disposable {
 }
 
 class WebviewProtocolProvider extends Disposable {
+
+	private _resolve!: () => void;
+	private _reject!: () => void;
+
+	public readonly ready: Promise<void>;
+
 	constructor(
 		handle: WebviewTagHandle,
-		private readonly _getExtensionLocation: () => URI | undefined,
-		private readonly _getLocalResourceRoots: () => ReadonlyArray<URI>,
-		private readonly _fileService: IFileService,
+		getExtensionLocation: () => URI | undefined,
+		getLocalResourceRoots: () => ReadonlyArray<URI>,
+		fileService: IFileService,
 	) {
 		super();
 
+		this.ready = new Promise((resolve, reject) => {
+			this._resolve = resolve;
+			this._reject = reject;
+		});
+
 		this._register(handle.onFirstLoad(contents => {
-			this.registerProtocols(contents);
+			registerFileProtocol(contents, WebviewResourceScheme, fileService, getExtensionLocation(), getLocalResourceRoots)
+				.then(this._resolve, this._reject);
 		}));
 	}
-
-	private registerProtocols(contents: WebContents) {
-		registerFileProtocol(contents, WebviewResourceScheme, this._fileService, this._getExtensionLocation(), () =>
-			this._getLocalResourceRoots()
-		);
-	}
 }
 
 class WebviewPortMappingProvider extends Disposable {
@@ -204,6 +210,7 @@ export class ElectronWebviewBasedWebview extends BaseWebview<WebviewTag> impleme
 	private _findStarted: boolean = false;
 
 	public extension: WebviewExtensionDescription | undefined;
+	private readonly _protocolProvider: WebviewProtocolProvider;
 
 	constructor(
 		id: string,
@@ -222,11 +229,12 @@ export class ElectronWebviewBasedWebview extends BaseWebview<WebviewTag> impleme
 		const webviewAndContents = this._register(new WebviewTagHandle(this.element!));
 		const session = this._register(new WebviewSession(webviewAndContents));
 
-		this._register(new WebviewProtocolProvider(
-			webviewAndContents,
-			() => this.extension ? this.extension.location : undefined,
-			() => (this.content.options.localResourceRoots || []),
-			fileService));
+		this._protocolProvider = new WebviewProtocolProvider
+			(webviewAndContents,
+				() => this.extension ? this.extension.location : undefined,
+				() => (this.content.options.localResourceRoots || []),
+				fileService);
+		this._register(this._protocolProvider);
 
 		this._register(new WebviewPortMappingProvider(
 			session,
@@ -322,7 +330,8 @@ export class ElectronWebviewBasedWebview extends BaseWebview<WebviewTag> impleme
 		parent.appendChild(this.element);
 	}
 
-	protected postMessage(channel: string, data?: any): void {
+	protected async postMessage(channel: string, data?: any): Promise<void> {
+		await this._protocolProvider.ready;
 		this.element?.send(channel, data);
 	}
 
diff --git a/src/vs/workbench/contrib/webview/electron-browser/webviewProtocols.ts b/src/vs/workbench/contrib/webview/electron-browser/webviewProtocols.ts
index e84ce9bf6fbcb..39e4349cb40ce 100644
--- a/src/vs/workbench/contrib/webview/electron-browser/webviewProtocols.ts
+++ b/src/vs/workbench/contrib/webview/electron-browser/webviewProtocols.ts
@@ -14,24 +14,27 @@ export function registerFileProtocol(
 	extensionLocation: URI | undefined,
 	getRoots: () => ReadonlyArray<URI>
 ) {
-	contents.session.protocol.registerBufferProtocol(protocol, async (request, callback: any) => {
-		try {
-			const result = await loadLocalResource(URI.parse(request.url), fileService, extensionLocation, getRoots);
-			if (result.type === WebviewResourceResponse.Type.Success) {
-				return callback({
-					data: Buffer.from(result.data.buffer),
-					mimeType: result.mimeType
-				});
+	return new Promise((resolve, reject) =>
+		contents.session.protocol.registerBufferProtocol(protocol, async (request, callback: any) => {
+			try {
+				const result = await loadLocalResource(URI.parse(request.url), fileService, extensionLocation, getRoots);
+				if (result.type === WebviewResourceResponse.Type.Success) {
+					return callback({
+						data: Buffer.from(result.data.buffer),
+						mimeType: result.mimeType
+					});
+				}
+				if (result.type === WebviewResourceResponse.Type.AccessDenied) {
+					console.error('Webview: Cannot load resource outside of protocol root');
+					return callback({ error: -10 /* ACCESS_DENIED: https://cs.chromium.org/chromium/src/net/base/net_error_list.h */ });
+				}
+			} catch  {
+				// noop
 			}
-			if (result.type === WebviewResourceResponse.Type.AccessDenied) {
-				console.error('Webview: Cannot load resource outside of protocol root');
-				return callback({ error: -10 /* ACCESS_DENIED: https://cs.chromium.org/chromium/src/net/base/net_error_list.h */ });
-			}
-		} catch  {
-			// noop
-		}
 
-		return callback({ error: -2 /* FAILED: https://cs.chromium.org/chromium/src/net/base/net_error_list.h */ });
-	});
+			return callback({ error: -2 /* FAILED: https://cs.chromium.org/chromium/src/net/base/net_error_list.h */ });
+		}, (err) => {
+			err ? reject(err) : resolve();
+		}));
 }