diff --git a/preview/PreviewCounter.cpp b/preview/PreviewCounter.cpp
index 56cd1540..9b880dd9 100644
--- a/preview/PreviewCounter.cpp
+++ b/preview/PreviewCounter.cpp
@@ -33,5 +33,6 @@ PreviewCounter &PreviewCounter::Period(const std::optional<std::chrono::microsec
 }
 
 std::optional<std::chrono::microseconds> PreviewCounter::GetPeriod() const {
+    std::unique_lock ul(m);
     return period;
 }