diff --git a/lib/ava-files.js b/lib/ava-files.js
index 243379d..dd9a2ee 100644
--- a/lib/ava-files.js
+++ b/lib/ava-files.js
@@ -13,11 +13,11 @@ function handlePaths(files, excludePatterns, globOptions) {
 	// Convert Promise to Bluebird
 	files = Promise.resolve(globby(files.concat(excludePatterns), globOptions));
 
-	const searchedParents = Object.create(null);
-	const foundFiles = Object.create(null);
+	const searchedParents = new Set();
+	const foundFiles = new Set();
 
 	function alreadySearchingParent(dir) {
-		if (searchedParents[dir]) {
+		if (searchedParents.has(dir)) {
 			return true;
 		}
 
@@ -40,7 +40,7 @@ function handlePaths(files, excludePatterns, globOptions) {
 					return null;
 				}
 
-				searchedParents[file] = true;
+				searchedParents.add(file);
 
 				let pattern = path.join(file, '**', '*.js');
 
@@ -67,8 +67,8 @@ function handlePaths(files, excludePatterns, globOptions) {
 		})
 		.map(file => path.resolve(file))
 		.filter(file => {
-			const alreadyFound = foundFiles[file];
-			foundFiles[file] = true;
+			const alreadyFound = foundFiles.has(file);
+			foundFiles.add(file);
 			return !alreadyFound;
 		});
 }
diff --git a/lib/process-adapter.js b/lib/process-adapter.js
index a319d74..2712b0c 100644
--- a/lib/process-adapter.js
+++ b/lib/process-adapter.js
@@ -62,7 +62,7 @@ if (debug.enabled) {
 	require('time-require'); // eslint-disable-line import/no-unassigned-import
 }
 
-const sourceMapCache = Object.create(null);
+const sourceMapCache = new Map();
 const cacheDir = opts.cacheDir;
 
 exports.installSourceMapSupport = () => {
@@ -70,10 +70,10 @@ exports.installSourceMapSupport = () => {
 		environment: 'node',
 		handleUncaughtExceptions: false,
 		retrieveSourceMap(source) {
-			if (sourceMapCache[source]) {
+			if (sourceMapCache.has(source)) {
 				return {
 					url: source,
-					map: fs.readFileSync(sourceMapCache[source], 'utf8')
+					map: fs.readFileSync(sourceMapCache.get(source), 'utf8')
 				};
 			}
 		}
@@ -85,7 +85,7 @@ exports.installPrecompilerHook = () => {
 		const precompiled = opts.precompiled[filename];
 
 		if (precompiled) {
-			sourceMapCache[filename] = path.join(cacheDir, precompiled + '.js.map');
+			sourceMapCache.set(filename, path.join(cacheDir, precompiled + '.js.map'));
 			return fs.readFileSync(path.join(cacheDir, precompiled + '.js'), 'utf8');
 		}