diff --git a/index.js b/index.js
index 3a1a2da..a1821f9 100644
--- a/index.js
+++ b/index.js
@@ -6,6 +6,9 @@ const electron = require('electron');
 
 const stat = promisify(fs.stat);
 
+// See https://cs.chromium.org/chromium/src/net/base/net_error_list.h
+const FILE_NOT_FOUND = -6;
+
 const getPath = async path_ => {
 	try {
 		const result = await stat(path_);
@@ -35,10 +38,15 @@ module.exports = options => {
 	const handler = async (request, callback) => {
 		const indexPath = path.join(options.directory, 'index.html');
 		const filePath = path.join(options.directory, decodeURIComponent(new URL(request.url).pathname));
+		const resolvedPath = await getPath(filePath);
 
-		callback({
-			path: (await getPath(filePath)) || indexPath
-		});
+		if (resolvedPath || !path.extname(filePath) || path.extname(filePath) === '.html') {
+			callback({
+				path: resolvedPath || indexPath
+			});
+		} else {
+			callback({error: FILE_NOT_FOUND});
+		}
 	};
 
 	if (electron.protocol.registerStandardSchemes) {
diff --git a/package.json b/package.json
index ffc1d9b..aa107f2 100644
--- a/package.json
+++ b/package.json
@@ -36,7 +36,7 @@
 		"url"
 	],
 	"devDependencies": {
-		"ava": "^1.4.1",
+		"ava": "^2.1.0",
 		"electron": "^3.0.0",
 		"spectron": "^5.0.0",
 		"tsd": "^0.7.3",
diff --git a/test/fixture-404-error.js b/test/fixture-404-error.js
new file mode 100644
index 0000000..5c65d2e
--- /dev/null
+++ b/test/fixture-404-error.js
@@ -0,0 +1,14 @@
+'use strict';
+const {app, BrowserWindow} = require('electron');
+const serve = require('..');
+
+serve({directory: __dirname});
+
+let mainWindow;
+
+(async () => {
+	await app.whenReady();
+
+	mainWindow = new BrowserWindow();
+	mainWindow.loadURL('app://-/subdir/page.pdf');
+})();
diff --git a/test/fixture-dir-fallback.js b/test/fixture-dir-fallback.js
new file mode 100644
index 0000000..3521683
--- /dev/null
+++ b/test/fixture-dir-fallback.js
@@ -0,0 +1,14 @@
+'use strict';
+const {app, BrowserWindow} = require('electron');
+const serve = require('..');
+
+serve({directory: __dirname});
+
+let mainWindow;
+
+(async () => {
+	await app.whenReady();
+
+	mainWindow = new BrowserWindow();
+	mainWindow.loadURL('app://-/subdir/page.html');
+})();
diff --git a/test/test.js b/test/test.js
index 0b90e85..e63f196 100644
--- a/test/test.js
+++ b/test/test.js
@@ -30,3 +30,27 @@ test('allows special characters in file paths', async t => {
 	await client.waitUntilTextExists('h1', '🚀', 5000);
 	t.pass();
 });
+
+test('fallbacks to root index if unresolved path has .html extension or no extension', async t => {
+	t.context.spectron = new Application({
+		path: electron,
+		args: ['fixture-dir-fallback.js']
+	});
+	await t.context.spectron.start();
+	const {client} = t.context.spectron;
+	await client.waitUntilWindowLoaded();
+	await client.waitUntilTextExists('h1', '🦄', 5000);
+	t.pass();
+});
+
+test('throws error if unresolved path has an extension other than .html', async t => {
+	t.context.spectron = new Application({
+		path: electron,
+		args: ['fixture-404-error.js']
+	});
+	await t.context.spectron.start();
+	const {client} = t.context.spectron;
+	await client.waitUntilWindowLoaded();
+	await t.throwsAsync(client.waitUntilTextExists('h1', '🦄', 5000));
+	t.pass();
+});