diff --git a/src/main/java/io/reactivex/internal/disposables/DisposableHelper.java b/src/main/java/io/reactivex/internal/disposables/DisposableHelper.java
index fa870957c0..564129a3d7 100644
--- a/src/main/java/io/reactivex/internal/disposables/DisposableHelper.java
+++ b/src/main/java/io/reactivex/internal/disposables/DisposableHelper.java
@@ -29,10 +29,21 @@ public enum DisposableHelper implements Disposable {
     DISPOSED
     ;
 
+    /**
+     * Checks if the given Disposable is the common {@link #DISPOSED} enum value.
+     * @param d the disposable to check
+     * @return true if d is {@link #DISPOSED}
+     */
     public static boolean isDisposed(Disposable d) {
         return d == DISPOSED;
     }
 
+    /**
+     * Atomically sets the field and disposes the old contents.
+     * @param field the target field
+     * @param d the new Disposable to set
+     * @return true if successful, false if the field contains the {@link #DISPOSED} instance.
+     */
     public static boolean set(AtomicReference<Disposable> field, Disposable d) {
         for (;;) {
             Disposable current = field.get();
@@ -144,6 +155,23 @@ public static void reportDisposableSet() {
         RxJavaPlugins.onError(new IllegalStateException("Disposable already set!"));
     }
 
+    /**
+     * Atomically tries to set the given Disposable on the field if it is null or disposes it if
+     * the field contains {@link #DISPOSED}.
+     * @param field the target field
+     * @param d the disposable to set
+     * @return true if successful, false otherwise
+     */
+    public static boolean trySet(AtomicReference<Disposable> field, Disposable d) {
+        if (!field.compareAndSet(null, d)) {
+            if (field.get() == DISPOSED) {
+                d.dispose();
+            }
+            return false;
+        }
+        return true;
+    }
+
     @Override
     public void dispose() {
         // deliberately no-op
diff --git a/src/main/java/io/reactivex/internal/operators/flowable/FlowableTimer.java b/src/main/java/io/reactivex/internal/operators/flowable/FlowableTimer.java
index e9cbc34922..cb7ac3e39f 100644
--- a/src/main/java/io/reactivex/internal/operators/flowable/FlowableTimer.java
+++ b/src/main/java/io/reactivex/internal/operators/flowable/FlowableTimer.java
@@ -36,7 +36,7 @@ public FlowableTimer(long delay, TimeUnit unit, Scheduler scheduler) {
 
     @Override
     public void subscribeActual(Subscriber<? super Long> s) {
-        IntervalOnceSubscriber ios = new IntervalOnceSubscriber(s);
+        TimerSubscriber ios = new TimerSubscriber(s);
         s.onSubscribe(ios);
 
         Disposable d = scheduler.scheduleDirect(ios, delay, unit);
@@ -44,7 +44,7 @@ public void subscribeActual(Subscriber<? super Long> s) {
         ios.setResource(d);
     }
 
-    static final class IntervalOnceSubscriber extends AtomicReference<Disposable>
+    static final class TimerSubscriber extends AtomicReference<Disposable>
     implements Subscription, Runnable {
 
         private static final long serialVersionUID = -2809475196591179431L;
@@ -53,7 +53,7 @@ static final class IntervalOnceSubscriber extends AtomicReference<Disposable>
 
         volatile boolean requested;
 
-        IntervalOnceSubscriber(Subscriber<? super Long> actual) {
+        TimerSubscriber(Subscriber<? super Long> actual) {
             this.actual = actual;
         }
 
@@ -74,16 +74,17 @@ public void run() {
             if (get() != DisposableHelper.DISPOSED) {
                 if (requested) {
                     actual.onNext(0L);
+                    lazySet(EmptyDisposable.INSTANCE);
                     actual.onComplete();
                 } else {
+                    lazySet(EmptyDisposable.INSTANCE);
                     actual.onError(new MissingBackpressureException("Can't deliver value due to lack of requests"));
                 }
-                lazySet(EmptyDisposable.INSTANCE);
             }
         }
 
         public void setResource(Disposable d) {
-            DisposableHelper.setOnce(this, d);
+            DisposableHelper.trySet(this, d);
         }
     }
 }
diff --git a/src/main/java/io/reactivex/internal/operators/observable/ObservableTimer.java b/src/main/java/io/reactivex/internal/operators/observable/ObservableTimer.java
index 3c15072190..242e77be8b 100644
--- a/src/main/java/io/reactivex/internal/operators/observable/ObservableTimer.java
+++ b/src/main/java/io/reactivex/internal/operators/observable/ObservableTimer.java
@@ -32,7 +32,7 @@ public ObservableTimer(long delay, TimeUnit unit, Scheduler scheduler) {
 
     @Override
     public void subscribeActual(Observer<? super Long> s) {
-        IntervalOnceObserver ios = new IntervalOnceObserver(s);
+        TimerObserver ios = new TimerObserver(s);
         s.onSubscribe(ios);
 
         Disposable d = scheduler.scheduleDirect(ios, delay, unit);
@@ -40,14 +40,14 @@ public void subscribeActual(Observer<? super Long> s) {
         ios.setResource(d);
     }
 
-    static final class IntervalOnceObserver extends AtomicReference<Disposable>
+    static final class TimerObserver extends AtomicReference<Disposable>
     implements Disposable, Runnable {
 
         private static final long serialVersionUID = -2809475196591179431L;
 
         final Observer<? super Long> actual;
 
-        IntervalOnceObserver(Observer<? super Long> actual) {
+        TimerObserver(Observer<? super Long> actual) {
             this.actual = actual;
         }
 
@@ -65,13 +65,13 @@ public boolean isDisposed() {
         public void run() {
             if (!isDisposed()) {
                 actual.onNext(0L);
-                actual.onComplete();
                 lazySet(EmptyDisposable.INSTANCE);
+                actual.onComplete();
             }
         }
 
         public void setResource(Disposable d) {
-            DisposableHelper.setOnce(this, d);
+            DisposableHelper.trySet(this, d);
         }
     }
 }
diff --git a/src/test/java/io/reactivex/internal/operators/flowable/FlowableTimerTest.java b/src/test/java/io/reactivex/internal/operators/flowable/FlowableTimerTest.java
index a28744c19c..c289b01293 100644
--- a/src/test/java/io/reactivex/internal/operators/flowable/FlowableTimerTest.java
+++ b/src/test/java/io/reactivex/internal/operators/flowable/FlowableTimerTest.java
@@ -13,9 +13,11 @@
 
 package io.reactivex.internal.operators.flowable;
 
+import static org.junit.Assert.*;
 import static org.mockito.ArgumentMatchers.*;
 import static org.mockito.Mockito.*;
 
+import java.util.List;
 import java.util.concurrent.TimeUnit;
 
 import org.junit.*;
@@ -25,6 +27,7 @@
 import io.reactivex.*;
 import io.reactivex.exceptions.*;
 import io.reactivex.flowables.ConnectableFlowable;
+import io.reactivex.plugins.RxJavaPlugins;
 import io.reactivex.schedulers.TestScheduler;
 import io.reactivex.subscribers.*;
 
@@ -324,4 +327,18 @@ public void run() {
             TestHelper.race(r1, r2);
         }
     }
+
+    @Test
+    public void timerDelayZero() {
+        List<Throwable> errors = TestHelper.trackPluginErrors();
+        try {
+            for (int i = 0; i < 1000; i++) {
+                Flowable.timer(0, TimeUnit.MILLISECONDS).blockingFirst();
+            }
+
+            assertTrue(errors.toString(), errors.isEmpty());
+        } finally {
+            RxJavaPlugins.reset();
+        }
+    }
 }
\ No newline at end of file
diff --git a/src/test/java/io/reactivex/internal/operators/observable/ObservableTimerTest.java b/src/test/java/io/reactivex/internal/operators/observable/ObservableTimerTest.java
index f307057cc2..5080d89f58 100644
--- a/src/test/java/io/reactivex/internal/operators/observable/ObservableTimerTest.java
+++ b/src/test/java/io/reactivex/internal/operators/observable/ObservableTimerTest.java
@@ -13,8 +13,10 @@
 
 package io.reactivex.internal.operators.observable;
 
+import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.*;
 
+import java.util.List;
 import java.util.concurrent.TimeUnit;
 
 import org.junit.*;
@@ -24,6 +26,7 @@
 import io.reactivex.exceptions.TestException;
 import io.reactivex.observables.ConnectableObservable;
 import io.reactivex.observers.*;
+import io.reactivex.plugins.RxJavaPlugins;
 import io.reactivex.schedulers.TestScheduler;
 
 public class ObservableTimerTest {
@@ -286,4 +289,18 @@ public void onComplete() {
     public void disposed() {
         TestHelper.checkDisposed(Observable.timer(1, TimeUnit.DAYS));
     }
+
+    @Test
+    public void timerDelayZero() {
+        List<Throwable> errors = TestHelper.trackPluginErrors();
+        try {
+            for (int i = 0; i < 1000; i++) {
+                Observable.timer(0, TimeUnit.MILLISECONDS).blockingFirst();
+            }
+
+            assertTrue(errors.toString(), errors.isEmpty());
+        } finally {
+            RxJavaPlugins.reset();
+        }
+    }
 }
\ No newline at end of file