diff --git a/docs/modules/ReadonlyNonEmptyArray.ts.md b/docs/modules/ReadonlyNonEmptyArray.ts.md
index 96fb90b6c..e673db26d 100644
--- a/docs/modules/ReadonlyNonEmptyArray.ts.md
+++ b/docs/modules/ReadonlyNonEmptyArray.ts.md
@@ -1376,7 +1376,7 @@ Added in v2.9.0
 ```ts
 export declare const union: <A>(
   E: Eq<A>
-) => (second: ReadonlyNonEmptyArray<A>) => (first: ReadonlyNonEmptyArray<A>) => ReadonlyNonEmptyArray<A>
+) => (second: readonly A[]) => (first: ReadonlyNonEmptyArray<A>) => ReadonlyNonEmptyArray<A>
 ```
 
 Added in v2.11.0
diff --git a/src/ReadonlyNonEmptyArray.ts b/src/ReadonlyNonEmptyArray.ts
index cce5f2713..87bed8e59 100644
--- a/src/ReadonlyNonEmptyArray.ts
+++ b/src/ReadonlyNonEmptyArray.ts
@@ -204,7 +204,7 @@ export const sortBy = <B>(
  */
 export const union = <A>(
   E: Eq<A>
-): ((second: ReadonlyNonEmptyArray<A>) => (first: ReadonlyNonEmptyArray<A>) => ReadonlyNonEmptyArray<A>) => {
+): ((second: ReadonlyArray<A>) => (first: ReadonlyNonEmptyArray<A>) => ReadonlyNonEmptyArray<A>) => {
   const uniqE = uniq(E)
   return (second) => (first) => uniqE(pipe(first, concat(second)))
 }
diff --git a/test/ReadonlyNonEmptyArray.ts b/test/ReadonlyNonEmptyArray.ts
index fad570f5e..0aa25bcf2 100644
--- a/test/ReadonlyNonEmptyArray.ts
+++ b/test/ReadonlyNonEmptyArray.ts
@@ -613,6 +613,11 @@ describe.concurrent('ReadonlyNonEmptyArray', () => {
     U.deepStrictEqual(concat([1, 2], [3, 4]), [1, 2, 3, 4])
     U.deepStrictEqual(concat([1, 2], [2, 3]), [1, 2, 3])
     U.deepStrictEqual(concat([1, 2], [1, 2]), [1, 2])
+
+    U.deepStrictEqual(_.union(N.Eq)([3, 4])([1, 2]), [1, 2, 3, 4])
+    U.deepStrictEqual(pipe([1, 2], _.union(N.Eq)([3, 4])), [1, 2, 3, 4])
+    U.deepStrictEqual(pipe([1, 2], _.union(N.Eq)([1, 3])), [1, 2, 3])
+    U.deepStrictEqual(pipe([1, 2], _.union(N.Eq)([])), [1, 2])
   })
 
   it('matchLeft', () => {