diff --git a/src/octoprint/util/__init__.py b/src/octoprint/util/__init__.py
index c16dbeef..ea4012a3 100644
--- a/src/octoprint/util/__init__.py
+++ b/src/octoprint/util/__init__.py
@@ -389,7 +389,11 @@ def silent_remove(file):
 
 
 def sanitize_ascii(line):
-	return unicode(line, 'ascii', 'replace').encode('ascii', 'replace').rstrip()
+	if not isinstance(line, basestring):
+		raise ValueError("Expected either str or unicode but got {} instead".format(line.__class__.__name__ if line is not None else None))
+	if isinstance(line, str):
+		line = unicode(line, 'ascii', 'replace')
+	return line.encode('ascii', 'replace').rstrip()
 
 
 def filter_non_ascii(line):
diff --git a/src/octoprint/util/comm.py b/src/octoprint/util/comm.py
index ab8cbea4..4062e743 100644
--- a/src/octoprint/util/comm.py
+++ b/src/octoprint/util/comm.py
@@ -1250,7 +1250,13 @@ class MachineCom(object):
 		if ret == '':
 			#self._log("Recv: TIMEOUT")
 			return ''
-		self._log("Recv: %s" % sanitize_ascii(ret))
+
+		try:
+			self._log("Recv: %s" % sanitize_ascii(ret))
+		except ValueError as e:
+			self._log("WARN: While reading last line: %s" % e)
+			self._log("Recv: %r" % ret)
+
 		return ret
 
 	def _getNext(self):