diff --git a/spring-statemachine-build-tests/src/test/java/org/springframework/statemachine/buildtests/TimerSmokeTests.java b/spring-statemachine-build-tests/src/test/java/org/springframework/statemachine/buildtests/TimerSmokeTests.java
index 99b6b710..3beeb7ce 100644
--- a/spring-statemachine-build-tests/src/test/java/org/springframework/statemachine/buildtests/TimerSmokeTests.java
+++ b/spring-statemachine-build-tests/src/test/java/org/springframework/statemachine/buildtests/TimerSmokeTests.java
@@ -55,6 +55,37 @@ public class TimerSmokeTests {
 		return builder.build();
 	}
 
+	private StateMachine<String, String> buildMachine2() throws Exception {
+
+		StateMachineBuilder.Builder<String, String> builder = StateMachineBuilder.builder();
+
+		builder.configureConfiguration()
+			.withConfiguration()
+				.taskExecutor(taskExecutor);
+
+		builder.configureStates()
+			.withStates()
+				.initial("initial").end("end").and()
+				.withStates().parent("initial").initial("inner");
+
+		builder.configureTransitions()
+			.withExternal()
+				.source("initial")
+				.target("end")
+				.timerOnce(30)
+				.and()
+			.withExternal()
+				.source("inner")
+				.target("end")
+				.timerOnce(15)
+				.and()
+			.withLocal()
+				.source("inner")
+				.event("repeate");
+
+		return builder.build();
+	}
+
 	@Test
 	public void testNPE() throws Exception {
 		StateMachine<String, String> stateMachine;
@@ -67,6 +98,21 @@ public class TimerSmokeTests {
 		}
 	}
 
+	@Test
+	public void testNPE2() throws Exception {
+
+		StateMachine<String, String> stateMachine;
+
+		for (int i = 0; i < 20; i++) {
+			stateMachine = buildMachine2();
+			stateMachine.start();
+			while(!stateMachine.isComplete()) {
+				stateMachine.sendEvent("repeate");
+			}
+			stateMachine.stop();
+		}
+	}
+
 	@Test
 	public void testDeadlock() throws Exception {
 		StateMachineTestPlan<String, String> plan;
diff --git a/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/AbstractStateMachine.java b/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/AbstractStateMachine.java
index 24a0c5c9..2a8aea9e 100644
--- a/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/AbstractStateMachine.java
+++ b/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/AbstractStateMachine.java
@@ -116,6 +116,7 @@ public abstract class AbstractStateMachine<S, E> extends StateMachineObjectSuppo
 	private volatile Message<E> forwardedInitialEvent;
 
 	private final Object lock = new Object();
+	private final Object lock2 = new Object();
 
 	private StateMachine<S, E> parentMachine;
 
@@ -211,30 +212,9 @@ public abstract class AbstractStateMachine<S, E> extends StateMachineObjectSuppo
 
 	@Override
 	public boolean sendEvent(Message<E> event) {
-		if (hasStateMachineError()) {
-			// TODO: should we throw exception?
-			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
-			return false;
+		synchronized (lock2) {
+			return sendEventInternal(event);
 		}
-
-		try {
-			event = getStateMachineInterceptors().preEvent(event, this);
-		} catch (Exception e) {
-			log.info("Event " + event + " threw exception in interceptors, not accepting event");
-			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
-			return false;
-		}
-
-		if (isComplete() || !isRunning()) {
-			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
-			return false;
-		}
-		boolean accepted = acceptEvent(event);
-		stateMachineExecutor.execute();
-		if (!accepted) {
-			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
-		}
-		return accepted;
 	}
 
 	@Override
@@ -555,6 +535,33 @@ public abstract class AbstractStateMachine<S, E> extends StateMachineObjectSuppo
 		forwardedInitialEvent = message;
 	}
 
+	private boolean sendEventInternal(Message<E> event) {
+		if (hasStateMachineError()) {
+			// TODO: should we throw exception?
+			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
+			return false;
+		}
+
+		try {
+			event = getStateMachineInterceptors().preEvent(event, this);
+		} catch (Exception e) {
+			log.info("Event " + event + " threw exception in interceptors, not accepting event");
+			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
+			return false;
+		}
+
+		if (isComplete() || !isRunning()) {
+			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
+			return false;
+		}
+		boolean accepted = acceptEvent(event);
+		stateMachineExecutor.execute();
+		if (!accepted) {
+			notifyEventNotAccepted(buildStateContext(Stage.EVENT_NOT_ACCEPTED, event, null, getRelayStateMachine(), getState(), null));
+		}
+		return accepted;
+	}
+
 	private StateMachine<S, E> getRelayStateMachine() {
 		return relay != null ? relay : this;
 	}
@@ -924,11 +931,18 @@ public abstract class AbstractStateMachine<S, E> extends StateMachineObjectSuppo
 		return null;
 	}
 
-	synchronized void setCurrentState(State<S, E> state, Message<E> message, Transition<S, E> transition, boolean exit, StateMachine<S, E> stateMachine) {
+	void setCurrentState(State<S, E> state, Message<E> message, Transition<S, E> transition, boolean exit, StateMachine<S, E> stateMachine) {
 		setCurrentState(state, message, transition, exit, stateMachine, null, null);
 	}
 
-	synchronized void setCurrentState(State<S, E> state, Message<E> message, Transition<S, E> transition, boolean exit,
+	void setCurrentState(State<S, E> state, Message<E> message, Transition<S, E> transition, boolean exit,
+			StateMachine<S, E> stateMachine, Collection<State<S, E>> sources, Collection<State<S, E>> targets) {
+		synchronized (lock2) {
+			setCurrentStateInternal(state, message, transition, exit, stateMachine, sources, targets);
+		}
+	}
+
+	private void setCurrentStateInternal(State<S, E> state, Message<E> message, Transition<S, E> transition, boolean exit,
 			StateMachine<S, E> stateMachine, Collection<State<S, E>> sources, Collection<State<S, E>> targets) {
 		State<S, E> findDeep = findDeepParent(state);
 		boolean isTargetSubOf = false;
diff --git a/spring-statemachine-core/src/test/java/org/springframework/statemachine/action/ActionAndTimerTests.java b/spring-statemachine-core/src/test/java/org/springframework/statemachine/action/ActionAndTimerTests.java
index 4ae0b9f2..b02c4c17 100644
--- a/spring-statemachine-core/src/test/java/org/springframework/statemachine/action/ActionAndTimerTests.java
+++ b/spring-statemachine-core/src/test/java/org/springframework/statemachine/action/ActionAndTimerTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 the original author or authors.
+ * Copyright 2016-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -59,6 +59,11 @@ public class ActionAndTimerTests extends AbstractStateMachineTests {
 		assertThat(testTimerAction.latch.await(4, TimeUnit.SECONDS), is(true));
 		assertThat(testTimerAction.e, nullValue());
 
+		// need to sleep for TimerTrigger not causing
+		// next event to get handled with threads, thus
+		// causing interrupt
+		Thread.sleep(1000);
+
 		machine.sendEvent(TestEvents.E2);
 		assertThat(testListener.s3EnteredLatch.await(2, TimeUnit.SECONDS), is(true));
 		assertThat(machine.getState().getIds(), containsInAnyOrder(TestStates.S3));
@@ -84,6 +89,11 @@ public class ActionAndTimerTests extends AbstractStateMachineTests {
 		assertThat(testTimerAction.latch.await(4, TimeUnit.SECONDS), is(true));
 		assertThat(testTimerAction.e, nullValue());
 
+		// need to sleep for TimerTrigger not causing
+		// next event to get handled with threads, thus
+		// causing interrupt
+		Thread.sleep(1000);
+
 		machine.sendEvent(TestEvents.E2);
 		assertThat(testListener.s3EnteredLatch.await(2, TimeUnit.SECONDS), is(true));
 		assertThat(machine.getState().getIds(), containsInAnyOrder(TestStates.S3));