diff --git a/tests/demux.spec.ts b/tests/demux.spec.ts
index aedbe6c..f01723a 100644
--- a/tests/demux.spec.ts
+++ b/tests/demux.spec.ts
@@ -172,6 +172,75 @@ test.cb("demux() should send input through correct pipeline using keyBy", t => {
     demuxed.end();
 });
 
+test("demux() should return false after if it has >= highWaterMark items buffered and drain should be emitted", t => {
+    return new Promise(async (resolve, reject) => {
+        t.plan(7);
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const input: Chunk[] = [
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+        ];
+        let pendingReads = input.length;
+        const highWaterMark = 5;
+        const slowProcessorSpeed = 25;
+        const construct = (destKey: string) => {
+            const first = map(
+                async (chunk: Chunk) => {
+                    await sleep(slowProcessorSpeed);
+                    return { ...chunk, mapped: [1] };
+                },
+                { highWaterMark: 1, objectMode: true },
+            );
+
+            // to clear first
+            first.on("data", chunk => {
+                expect(chunk.mapped).to.deep.equal([1]);
+                pendingReads--;
+                if (pendingReads === 0) {
+                    resolve();
+                }
+                t.pass();
+            });
+
+            return first;
+        };
+
+        const _demux = demux(
+            construct,
+            { key: "key" },
+            {
+                objectMode: true,
+                highWaterMark,
+            },
+        );
+
+        _demux.on("error", err => {
+            reject();
+        });
+
+        for (const item of input) {
+            const res = _demux.write(item);
+            expect(_demux._writableState.length).to.be.at.most(highWaterMark);
+            if (!res) {
+                await new Promise((resolv, rej) => {
+                    _demux.once("drain", () => {
+                        expect(_demux._writableState.length).to.be.equal(0);
+                        t.pass();
+                        resolv();
+                    });
+                });
+            }
+        }
+    });
+});
+
 test("demux() when write returns false, drain event should be emitted after at least slowProcessorSpeed * highWaterMark", t => {
     return new Promise(async (resolve, reject) => {
         t.plan(7);
@@ -211,12 +280,7 @@ test("demux() when write returns false, drain event should be emitted after at l
                 { highWaterMark: 1, objectMode: true },
             );
 
-            const second = map(async (chunk: Chunk) => {
-                chunk.mapped.push(2);
-                return chunk;
-            });
-
-            first.pipe(second).pipe(sink);
+            first.pipe(sink);
             return first;
         };
         const _demux = demux(
@@ -231,20 +295,18 @@ test("demux() when write returns false, drain event should be emitted after at l
             reject();
         });
 
-        let start = null;
+        let start = performance.now();
         for (const item of input) {
             const res = _demux.write(item);
-            expect(_demux._writableState.length).to.be.at.most(highWaterMark);
             if (!res) {
-                start = performance.now();
                 await new Promise((resolv, rej) => {
                     _demux.once("drain", () => {
-                        expect(_demux._writableState.length).to.be.equal(0);
                         expect(performance.now() - start).to.be.greaterThan(
                             slowProcessorSpeed * highWaterMark,
                         );
                         t.pass();
                         resolv();
+                        start = performance.now();
                     });
                 });
             }