diff --git a/package.json b/package.json
index dd64dcd..258ca79 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "iso-fns",
   "description": "A string-based date-time library for Javascript.",
-  "version": "2.0.0-alpha.20",
+  "version": "2.0.0-alpha.21",
   "main": "./cjs/index.js",
   "module": "./esm/index.js",
   "exports": {
diff --git a/src/fns/instantFns.ts b/src/fns/instantFns.ts
index c67e91a..4fcf69a 100644
--- a/src/fns/instantFns.ts
+++ b/src/fns/instantFns.ts
@@ -179,7 +179,7 @@ export const instantFns: IInstantFns = {
   },
   formatISO9075(instant) {
     if (!ES.IsTemporalInstant(instant)) throw new TypeError('invalid receiver')
-    return instant.slice(0, -1).replace('T', ' ') as Iso.DateTime
+    return instant.slice(0, -1).replace('T', ' ')
   },
   toJsDate(instant) {
     return new Date(instant)
diff --git a/src/types/IInstantFns.ts b/src/types/IInstantFns.ts
index 7193c6c..7c46d48 100644
--- a/src/types/IInstantFns.ts
+++ b/src/types/IInstantFns.ts
@@ -89,7 +89,7 @@ export interface IInstantFns {
   isEqualOrBefore(instant: Iso.Instant, other: Iso.Instant): boolean
   isEqualOrAfter(instant: Iso.Instant, other: Iso.Instant): boolean
   toZonedDateTime(instant: Iso.Instant, timeZone: string): Iso.ZonedDateTime
-  formatISO9075(instant: Iso.Instant): Iso.DateTime
+  formatISO9075(instant: Iso.Instant): string
   toJsDate(instant: Iso.Instant): Date
   fromEpochSeconds(epochSeconds: number): Iso.Instant
   from(item: any): Iso.Instant