diff --git a/src/datetime.js b/src/datetime.js
index 85e249fb8..3080fc7fc 100644
--- a/src/datetime.js
+++ b/src/datetime.js
@@ -1679,7 +1679,10 @@ export default class DateTime {
    */
   diff(otherDateTime, unit = "milliseconds", opts = {}) {
     if (!this.isValid || !otherDateTime.isValid) {
-      return Duration.invalid(this.invalid || otherDateTime.invalid);
+      return Duration.invalid(
+        this.invalid || otherDateTime.invalid,
+        "created by diffing an invalid DateTime"
+      );
     }
 
     const durOpts = Object.assign(
diff --git a/test/datetime/relative.test.js b/test/datetime/relative.test.js
index 3a9196231..61c34003b 100644
--- a/test/datetime/relative.test.js
+++ b/test/datetime/relative.test.js
@@ -43,14 +43,14 @@ test("DateTime#toRelative takes a round argument", () => {
 });
 
 test("DateTime#toRelative takes a unit argument", () => {
-  const base = DateTime.fromObject({ year: 1983, month: 10, day: 14 });
+  const base = DateTime.fromObject({ year: 2018, month: 10, day: 14, zone: "UTC" });
   expect(base.plus({ months: 15 }).toRelative({ base, unit: "months" })).toBe("in 15 months");
   expect(base.minus({ months: 15 }).toRelative({ base, unit: "months" })).toBe("15 months ago");
-  expect(base.plus({ months: 6 }).toRelative({ base, unit: "years", round: false })).toBe(
-    "in 0.5 years"
+  expect(base.plus({ months: 3 }).toRelative({ base, unit: "years", round: false })).toBe(
+    "in 0.25 years"
   );
-  expect(base.minus({ months: 6 }).toRelative({ base, unit: "years", round: false })).toBe(
-    "0.5 years ago"
+  expect(base.minus({ months: 3 }).toRelative({ base, unit: "years", round: false })).toBe(
+    "0.25 years ago"
   );
 });