diff --git a/lib/arguments/file-url.js b/lib/arguments/file-url.js
index 0a0bde317..e7a8560b5 100644
--- a/lib/arguments/file-url.js
+++ b/lib/arguments/file-url.js
@@ -7,7 +7,7 @@ export const safeNormalizeFileUrl = (file, name) => {
 	const isString = typeof fileString === 'string'
 		// In Deno node:process execPath is a special object, not just a string:
 		// https://github.com/denoland/deno/blob/f460188e583f00144000aa0d8ade08218d47c3c1/ext/node/polyfills/process.ts#L344
-		|| Object.getPrototypeOf(fileString) === String.prototype;
+		|| (fileString && Object.getPrototypeOf(fileString) === String.prototype);
 
 	if (!isString) {
 		throw new TypeError(`${name} must be a string or a file URL: ${fileString}.`);