diff --git a/codestyle.py b/codestyle.py
--- a/codestyle.py
+++ b/codestyle.py
@@ -46,3 +46,5 @@
     else:
         stderr.write("The `pycodestyle` (previously `pep8`) module is not installed.")
         stderr.flush()
+        stdout.write("{0:>10}".format(0))
+        stdout.flush()
diff --git a/pythonstylechecking.cpp b/pythonstylechecking.cpp
--- a/pythonstylechecking.cpp
+++ b/pythonstylechecking.cpp
@@ -67,12 +67,14 @@
         auto serverPath = QStandardPaths::locate(QStandardPaths::GenericDataLocation, "kdevpythonsupport/codestyle.py");
         if ( serverPath.isEmpty() ) {
             qWarning() << "setup problem: codestyle.py not found";
+            m_mutex.unlock();
             return;
         }
         m_checkerProcess.start(python, {serverPath});
         m_checkerProcess.waitForStarted(30);
         if ( m_checkerProcess.state() != QProcess::Running ) {
             qWarning() << "failed to start code checker process";
+            m_mutex.unlock();
             return;
         }
     }