diff --git a/src/core/FS.ts b/src/core/FS.ts
index 5120e7a5..5e62a9bd 100644
--- a/src/core/FS.ts
+++ b/src/core/FS.ts
@@ -77,7 +77,8 @@ function normalizePath(p: string): string {
  * @hidden
  */
 function normalizeOptions(options: any, defEnc: string | null, defFlag: string, defMode: number | null): {encoding: string; flag: string; mode: number} {
-  switch (typeof options) {
+  // typeof null === 'object' so special-case handing is needed.
+  switch (options === null ? 'null' : typeof options) {
     case 'object':
       return {
         encoding: typeof options['encoding'] !== 'undefined' ? options['encoding'] : defEnc,
@@ -90,12 +91,16 @@ function normalizeOptions(options: any, defEnc: string | null, defFlag: string,
         flag: defFlag,
         mode: defMode!
       };
-    default:
+    case 'null':
+    case 'undefined':
+    case 'function':
       return {
         encoding: defEnc!,
         flag: defFlag,
         mode: defMode!
       };
+    default:
+      throw new TypeError(`"options" must be a string or an object, got ${typeof options} instead.`);
   }
 }