diff --git a/types/seamless-immutable/index.d.ts b/types/seamless-immutable/index.d.ts
index 1206034846..ebf546d409 100644
--- a/types/seamless-immutable/index.d.ts
+++ b/types/seamless-immutable/index.d.ts
@@ -105,7 +105,7 @@ declare namespace SeamlessImmutable {
             map<TTarget>(mapFuction: (item: T) => TTarget): Immutable<TTarget[]>;
             filter(filterFunction: (item: T) => boolean): Immutable<T[]>;
             slice(start?: number, end?: number): Immutable<T[]>;
-            concat(...arr: T[]): Immutable<T[]>;
+            concat(...arr: Array<T|T[]>): Immutable<T[]>;
             reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): Immutable<T>;
             reduce<TTarget>(callbackfn: (previousValue: TTarget, currentValue: T, currentIndex: number, array: T[]) => TTarget, initialValue?: TTarget): Immutable<TTarget>;
             reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): Immutable<T>;
diff --git a/types/seamless-immutable/seamless-immutable-tests.ts b/types/seamless-immutable/seamless-immutable-tests.ts
index 1e54f9eff6..b44dc58543 100644
--- a/types/seamless-immutable/seamless-immutable-tests.ts
+++ b/types/seamless-immutable/seamless-immutable-tests.ts
@@ -120,7 +120,14 @@ interface ExtendedUser extends User {
 
     // concat. Call the mutable array's 'concat' with the same args to ensure compatability. Make sure the output array is immutable.
     array.asMutable().concat({ firstName: 'Happy', lastName: 'Cat' });
-    const concat: Immutable.Immutable<User[]> = array.concat({ firstName: 'Happy', lastName: 'Cat' });
+    const concat: Immutable.Immutable<User[]> = array
+        .concat({ firstName: 'Happy', lastName: 'Cat' }, { firstName: 'Silly', lastName: 'Cat' })
+        .concat([{firstName: 'saucy', lastName: 'Cat'}], {firstName: 'Fussy', lastName: 'Cat'})
+        .concat([
+            {firstName: 'Fancy', lastName: 'Cat'},
+            {firstName: 'Sassy', lastName: 'Cat'}],
+            [{firstName: 'Bossy', lastName: 'Cat'}
+        ]);
     concat.asMutable();
 
     // reduce. Call the mutable array's 'reduce' with the same function to ensure compatability. Make sure the output array is immutable.