diff --git a/src/octoprint/printer.py b/src/octoprint/printer.py
index f992b78d..fd48b743 100644
--- a/src/octoprint/printer.py
+++ b/src/octoprint/printer.py
@@ -677,6 +677,7 @@ class StateMonitor(object):
 		self._offsets = {}
 
 		self._changeEvent = threading.Event()
+		self._stateMutex = threading.Lock()
 
 		self._lastUpdate = time.time()
 		self._worker = threading.Thread(target=self._work)
@@ -706,8 +707,9 @@ class StateMonitor(object):
 		self._changeEvent.set()
 
 	def setState(self, state):
-		self._state = state
-		self._changeEvent.set()
+		with self._stateMutex:
+			self._state = state
+			self._changeEvent.set()
 
 	def setJobData(self, jobData):
 		self._jobData = jobData
@@ -725,16 +727,17 @@ class StateMonitor(object):
 		while True:
 			self._changeEvent.wait()
 
-			now = time.time()
-			delta = now - self._lastUpdate
-			additionalWaitTime = self._ratelimit - delta
-			if additionalWaitTime > 0:
-				time.sleep(additionalWaitTime)
+			with self._stateMutex:
+				now = time.time()
+				delta = now - self._lastUpdate
+				additionalWaitTime = self._ratelimit - delta
+				if additionalWaitTime > 0:
+					time.sleep(additionalWaitTime)
 
-			data = self.getCurrentData()
-			self._updateCallback(data)
-			self._lastUpdate = time.time()
-			self._changeEvent.clear()
+				data = self.getCurrentData()
+				self._updateCallback(data)
+				self._lastUpdate = time.time()
+				self._changeEvent.clear()
 
 	def getCurrentData(self):
 		return {
diff --git a/src/octoprint/util/comm.py b/src/octoprint/util/comm.py
index a15eb4de..98cb0eb9 100644
--- a/src/octoprint/util/comm.py
+++ b/src/octoprint/util/comm.py
@@ -340,11 +340,11 @@ class MachineCom(object):
 	def close(self, isError = False):
 		printing = self.isPrinting() or self.isPaused()
 		if self._serial is not None:
-			self._serial.close()
 			if isError:
 				self._changeState(self.STATE_CLOSED_WITH_ERROR)
 			else:
 				self._changeState(self.STATE_CLOSED)
+			self._serial.close()
 		self._serial = None
 
 		if settings().get(["feature", "sdSupport"]):