diff --git a/packages/defmulti/README.md b/packages/defmulti/README.md
index f8ad7219cb..0e6e55e5c0 100644
--- a/packages/defmulti/README.md
+++ b/packages/defmulti/README.md
@@ -110,9 +110,9 @@ foo.rels();
 Returns a multi-dispatch function which delegates to one of the provided
 implementations, based on the arity (number of args) when the function
 is called. Internally uses `defmulti`, so new arities can be dynamically
-added (or removed) at a later time. `defmultiN` also registers a
-`DEFAULT` implementation which simply throws an `IllegalArityError` when
-invoked.
+added (or removed) at a later time. If no `fallback` is provided,
+`defmultiN` also registers a `DEFAULT` implementation which simply
+throws an `IllegalArityError` when invoked.
 
 **Note:** Unlike `defmulti` no argument type checking is supported,
 however you can specify the return type for the generated function.
diff --git a/packages/defmulti/src/index.ts b/packages/defmulti/src/index.ts
index f355416f95..202714b8f7 100644
--- a/packages/defmulti/src/index.ts
+++ b/packages/defmulti/src/index.ts
@@ -215,9 +215,10 @@ const makeRels = (spec: AncestorDefs) => {
  * Returns a multi-dispatch function which delegates to one of the
  * provided implementations, based on the arity (number of args) when
  * the function is called. Internally uses `defmulti`, so new arities
- * can be dynamically added (or removed) at a later time. `defmultiN`
- * also registers a `DEFAULT` implementation which simply throws an
- * `IllegalArityError` when invoked.
+ * can be dynamically added (or removed) at a later time. If no
+ * `fallback` is provided, `defmultiN` also registers a `DEFAULT`
+ * implementation which simply throws an `IllegalArityError` when
+ * invoked.
  *
  * **Note:** Unlike `defmulti` no argument type checking is supported,
  * however you can specify the return type for the generated function.
@@ -244,10 +245,14 @@ const makeRels = (spec: AncestorDefs) => {
  * ```
  *
  * @param impls
+ * @param fallback
  */
-export function defmultiN<T>(impls: { [id: number]: Implementation<T> }) {
+export function defmultiN<T>(
+    impls: { [id: number]: Implementation<T> },
+    fallback?: Implementation<T>
+) {
     const fn = defmulti<T>((...args: any[]) => args.length);
-    fn.add(DEFAULT, (...args) => illegalArity(args.length));
+    fn.add(DEFAULT, fallback || ((...args) => illegalArity(args.length)));
     for (let id in impls) {
         fn.add(id, impls[id]);
     }