diff --git a/Sources/Operators/ConcatMap.swift b/Sources/Operators/ConcatMap.swift
index c2f31ca..196036e 100644
--- a/Sources/Operators/ConcatMap.swift
+++ b/Sources/Operators/ConcatMap.swift
@@ -112,13 +112,12 @@ private extension Publishers.ConcatMap {
 
         override func receive(_ input: Upstream.Output) -> Subscribers.Demand {
             let mapped = transform(input)
-
             lock.lock()
+            defer { lock.unlock() }
+
             if activePublisher == nil {
-                lock.unlock()
                 setActivePublisher(mapped)
             } else {
-                lock.unlock()
                 bufferedPublishers.append(mapped)
             }
 
@@ -132,15 +131,15 @@ private extension Publishers.ConcatMap {
 
             publisher.sink(
                 receiveCompletion: { completion in
+                    self.lock.lock()
+                    defer { self.lock.unlock() }
                     switch completion {
                     case .finished:
-                        self.lock.lock()
                         guard let next = self.bufferedPublishers.first else {
-                            self.lock.unlock()
+                            self.activePublisher = nil
                             return
                         }
                         self.bufferedPublishers.removeFirst()
-                        self.lock.unlock()
                         self.setActivePublisher(next)
                     case .failure(let error):
                         self.receive(completion: .failure(error))
diff --git a/Tests/ConcatMapTests.swift b/Tests/ConcatMapTests.swift
index d7c3259..6c7bef3 100644
--- a/Tests/ConcatMapTests.swift
+++ b/Tests/ConcatMapTests.swift
@@ -24,9 +24,9 @@ final class ConcatMapTests: XCTestCase {
         cancellables = []
     }
 
-    func test_publishes_values_inOrder() {
+    func test_publishes_values_in_order() {
         var receivedValues = [Int]()
-        let expectedValues = [1, 2, 4, 5, 6]
+        let expectedValues = [1, 2, 3]
 
         let firstPublisher = P()
         let secondPublisher = P()
@@ -43,19 +43,89 @@ final class ConcatMapTests: XCTestCase {
 
         sut.send(firstPublisher)
         sut.send(secondPublisher)
+
+        firstPublisher.send(1)
+        firstPublisher.send(completion: .finished)
+
+        secondPublisher.send(2)
         sut.send(thirdPublisher)
+        secondPublisher.send(completion: .finished)
+
+        thirdPublisher.send(3)
+
+        XCTAssertEqual(expectedValues, receivedValues)
+    }
+
+    func test_ignores_values_of_subsequent_while_previous_hasNot_completed() {
+        var receivedValues = [Int]()
+        let expectedValues = [1, 3]
+
+        let firstPublisher = P()
+        let secondPublisher = P()
+
+        let sut = PassthroughSubject<P, TestError>()
+
+        sut.concatMap { $0 }
+            .sink(
+                receiveCompletion: { _ in },
+                receiveValue: { value in receivedValues.append(value) }
+            )
+            .store(in: &cancellables)
+
+        sut.send(firstPublisher)
+        sut.send(secondPublisher)
 
         firstPublisher.send(1)
-        firstPublisher.send(2)
-        // values sent onto the second publisher will be ignored as long as the first publisher hasn't completed
+        secondPublisher.send(2)
+        firstPublisher.send(completion: .finished)
+
         secondPublisher.send(3)
+        secondPublisher.send(completion: .finished)
+
+        XCTAssertEqual(expectedValues, receivedValues)
+    }
+
+    func test_publishes_values_of_subsequent_publisher_after_emptying_publisher_queue() {
+        var receivedValues = [Int]()
+        let expectedValues = [1, 2]
+
+        let firstPublisher = P()
+        let secondPublisher = P()
+
+        let sut = PassthroughSubject<P, TestError>()
+
+        sut.concatMap { $0 }
+            .sink(
+                receiveCompletion: { _ in },
+                receiveValue: { value in receivedValues.append(value) }
+            )
+            .store(in: &cancellables)
+
+        sut.send(firstPublisher)
+        firstPublisher.send(1)
         firstPublisher.send(completion: .finished)
 
-        secondPublisher.send(4)
-        secondPublisher.send(5)
+        sut.send(secondPublisher)
+        secondPublisher.send(2)
         secondPublisher.send(completion: .finished)
 
-        thirdPublisher.send(6)
+        XCTAssertEqual(expectedValues, receivedValues)
+    }
+
+    func test_synchronous_completion() {
+        var receivedValues = [Int]()
+        let expectedValues = [1, 2]
+        let firstPublisher = Just<Int>(1)
+        let secondPublisher = Just<Int>(2)
+
+        let sut = PassthroughSubject<Just<Int>, Never>()
+
+        sut.concatMap { $0 }
+            .sink { value in receivedValues.append(value) }
+            .store(in: &cancellables)
+
+        sut.send(firstPublisher)
+        sut.send(secondPublisher)
 
         XCTAssertEqual(expectedValues, receivedValues)
     }