diff --git a/src/duration.js b/src/duration.js
index 49033a47..56cf194b 100644
--- a/src/duration.js
+++ b/src/duration.js
@@ -462,6 +462,8 @@ export default class Duration {
    * ```
    */
   toHuman(opts = {}) {
+    if (!this.isValid) return INVALID;
+
     const l = orderedUnits
       .map((unit) => {
         const val = this.values[unit];
@@ -576,9 +578,11 @@ export default class Duration {
    * @return {number}
    */
   toMillis() {
+    if (!this.isValid) return NaN;
+
     let sum = this.values.milliseconds ?? 0;
     for (let unit of reverseUnits.slice(1)) {
-      if (this.values?.[unit]) {
+      if (this.values[unit]) {
         sum += this.values[unit] * this.matrix[unit]["milliseconds"];
       }
     }
diff --git a/test/duration/invalid.test.js b/test/duration/invalid.test.js
index 98f5e920..ffde586a 100644
--- a/test/duration/invalid.test.js
+++ b/test/duration/invalid.test.js
@@ -31,3 +31,23 @@ test("Diffing invalid DateTimes creates invalid Durations", () => {
 test("Duration.invalid produces invalid Intervals", () => {
   expect(Duration.invalid("because").isValid).toBe(false);
 });
+
+test("Duration.toMillis produces NaN on invalid Durations", () => {
+  expect(Duration.invalid("because").toMillis()).toBe(NaN);
+});
+
+test("Duration.as produces NaN on invalid Durations", () => {
+  expect(Duration.invalid("because").as("seconds")).toBe(NaN);
+});
+
+test("Duration.toHuman produces null on invalid Durations", () => {
+  expect(Duration.invalid("because").toHuman()).toBe("Invalid Duration");
+});
+
+test("Duration.toISO produces null on invalid Durations", () => {
+  expect(Duration.invalid("because").toISO()).toBeNull();
+});
+
+test("Duration.toFormat produces Invalid Duration on invalid Durations", () => {
+  expect(Duration.invalid("because").toFormat("s")).toBe("Invalid Duration");
+});