diff --git a/src/core.ts b/src/core.ts
index 33de308..a4cde16 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -1647,11 +1647,12 @@ export class Stream<T> implements InternalListener<T> {
    *
    * @return {Stream}
    */
-  flattenConcurrently<R, T extends Stream<R>>(): T {
+  flattenConcurrently<R>(): T {
     const p = this._prod;
-    return <T> new Stream<R>(p instanceof MapOperator || p instanceof FilterMapOperator ?
-      new MapFlattenConcOperator(<MapOperator<R, Stream<R>>> <any> p) :
-      new FlattenConcOperator(<Stream<Stream<R>>> <any> this)
+    return <T> <any> new Stream<R>(
+      p instanceof MapOperator || p instanceof FilterMapOperator ?
+        new MapFlattenConcOperator(<MapOperator<R, Stream<R>>> <any> p) :
+        new FlattenConcOperator(<Stream<Stream<R>>> <any> this)
     );
   }
 
diff --git a/tests/operator/flattenConcurrently.ts b/tests/operator/flattenConcurrently.ts
index 1684f86..4990738 100644
--- a/tests/operator/flattenConcurrently.ts
+++ b/tests/operator/flattenConcurrently.ts
@@ -1,14 +1,15 @@
-import xs from '../../src/index';
+import xs, {Stream, Listener} from '../../src/index';
 import * as assert from 'assert';
 
 describe('Stream.prototype.flattenConcurrently', () => {
   describe('with map', () => {
     it('should expand each periodic event with 3 sync events', (done) => {
       const stream = xs.periodic(100).take(3)
-      .map(i => xs.of(1 + i, 2 + i, 3 + i))
-      .flattenConcurrently();
+        .map(i => xs.of(1 + i, 2 + i, 3 + i))
+        .flattenConcurrently();
       const expected = [1, 2, 3, 2, 3, 4, 3, 4, 5];
-      const listener = {
+
+      stream.addListener({
         next: (x: number) => {
           assert.equal(x, expected.shift());
         },
@@ -17,8 +18,24 @@ describe('Stream.prototype.flattenConcurrently', () => {
           assert.equal(expected.length, 0);
           done();
         },
-      };
-      stream.addListener(listener);
+      });
+    });
+
+    it('should return a flat stream with correct TypeScript types', (done) => {
+      const streamStrings: Stream<string> = Stream.create({
+        start: (listener: Listener<string>) => {},
+        stop: () => {}
+      });
+
+      const streamBooleans: Stream<boolean> = Stream.create({
+        start: (listener: Listener<boolean>) => {},
+        stop: () => {}
+      });
+
+      // Type checked by the compiler. Without Stream<boolean> it does not compile.
+      const flat: Stream<boolean> = streamStrings.map(x => streamBooleans)
+        .flattenConcurrently();
+      done();
     });
 
     it('should expand 3 sync events as a periodic each', (done) => {
@@ -30,8 +47,9 @@ describe('Stream.prototype.flattenConcurrently', () => {
       // -------10------11
       // -----------20----------21
       const expected = ['00', '01', '10', '20', '11', '21'];
-      const listener = {
-        next: (x: number) => {
+
+      stream.addListener({
+        next: (x: string) => {
           assert.equal(x, expected.shift());
         },
         error: (err: any) => done(err),
@@ -39,8 +57,7 @@ describe('Stream.prototype.flattenConcurrently', () => {
           assert.equal(expected.length, 0);
           done();
         },
-      };
-      stream.addListener(listener);
+      });
     });
 
     it('should expand 3 async events as a periodic each', (done) => {
@@ -54,8 +71,9 @@ describe('Stream.prototype.flattenConcurrently', () => {
       //      ----10--11--12
       //           ------------20-----------21----------22
       const expected = ['00', '01', '10', '02', '11', '12', '20', '21', '22'];
+
       stream.addListener({
-        next: (x: number) => {
+        next: (x: string) => {
           assert.equal(x, expected.shift());
         },
         error: (err: any) => done(err),
@@ -79,8 +97,9 @@ describe('Stream.prototype.flattenConcurrently', () => {
       //           ------------20-----------21----------22
 
       const expected = ['00', '01', '10', '02', '11', '12', '20', '21', '22'];
-      const listener = {
-        next: (x: number) => {
+
+      stream.addListener({
+        next: (x: string) => {
           assert.equal(x, expected.shift());
         },
         error: (err: any) => done(err),
@@ -88,8 +107,7 @@ describe('Stream.prototype.flattenConcurrently', () => {
           assert.equal(expected.length, 0);
           done();
         }
-      };
-      stream.addListener(listener);
+      });
     });
 
     it('should propagate user mistakes in project as errors', (done) => {