diff --git a/src/index.js b/src/index.js
index f50b59e78..8c919b0d8 100644
--- a/src/index.js
+++ b/src/index.js
@@ -251,11 +251,12 @@ class Dayjs {
   }
 
   format(formatStr) {
-    if (!this.isValid()) return C.INVALID_DATE_STRING
+    const locale = this.$locale()
+
+    if (!this.isValid()) return locale.invalidDate || C.INVALID_DATE_STRING
 
     const str = formatStr || C.FORMAT_DEFAULT
     const zoneStr = Utils.z(this)
-    const locale = this.$locale()
     const { $H, $m, $M } = this
     const {
       weekdays, months, meridiem
diff --git a/test/plugin/updateLocale.test.js b/test/plugin/updateLocale.test.js
index ecbcbd41c..1fdfe93cb 100644
--- a/test/plugin/updateLocale.test.js
+++ b/test/plugin/updateLocale.test.js
@@ -68,4 +68,15 @@ describe('Update locale', () => {
     expect(dayjs().format(formatString))
       .toEqual(moment().format(formatString))
   })
+
+  it('Update invalid date string', () => {
+    const locale = 'en'
+    const localeSetting = { invalidDate: 'bad date' }
+    dayjs.updateLocale(locale, localeSetting)
+    moment.updateLocale(locale, localeSetting)
+    dayjs.locale(locale)
+    moment.locale(locale)
+    expect(dayjs('').format()).toBe(moment('').format())
+    expect(dayjs('otherString').format()).toBe(moment('otherString').format())
+  })
 })