diff --git a/wpilibc/src/main/native/cpp/Notifier.cpp b/wpilibc/src/main/native/cpp/Notifier.cpp
index a7fa0381bd..3065e655cd 100644
--- a/wpilibc/src/main/native/cpp/Notifier.cpp
+++ b/wpilibc/src/main/native/cpp/Notifier.cpp
@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2008-2019 FIRST. All Rights Reserved.                        */
+/* Copyright (c) 2008-2020 FIRST. All Rights Reserved.                        */
 /* Open Source Software - may be modified and shared by FRC teams. The code   */
 /* must be accompanied by the FIRST BSD license file in the root directory of */
 /* the project.                                                               */
@@ -129,6 +129,8 @@ void Notifier::StartPeriodic(units::second_t period) {
 }
 
 void Notifier::Stop() {
+  std::scoped_lock lock(m_processMutex);
+  m_periodic = false;
   int32_t status = 0;
   HAL_CancelNotifierAlarm(m_notifier, &status);
   wpi_setHALError(status);
diff --git a/wpilibj/src/main/java/edu/wpi/first/wpilibj/Notifier.java b/wpilibj/src/main/java/edu/wpi/first/wpilibj/Notifier.java
index 806bfa9899..546da5b836 100644
--- a/wpilibj/src/main/java/edu/wpi/first/wpilibj/Notifier.java
+++ b/wpilibj/src/main/java/edu/wpi/first/wpilibj/Notifier.java
@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2016-2019 FIRST. All Rights Reserved.                        */
+/* Copyright (c) 2016-2020 FIRST. All Rights Reserved.                        */
 /* Open Source Software - may be modified and shared by FRC teams. The code   */
 /* must be accompanied by the FIRST BSD license file in the root directory of */
 /* the project.                                                               */
@@ -207,6 +207,12 @@ public class Notifier implements AutoCloseable {
    * function will block until the handler call is complete.
    */
   public void stop() {
-    NotifierJNI.cancelNotifierAlarm(m_notifier.get());
+    m_processLock.lock();
+    try {
+      m_periodic = false;
+      NotifierJNI.cancelNotifierAlarm(m_notifier.get());
+    } finally {
+      m_processLock.unlock();
+    }
   }
 }