diff --git a/common/ModuleSummation.cpp b/common/ModuleSummation.cpp
index 134b3ac9..50866ae7 100644
--- a/common/ModuleSummation.cpp
+++ b/common/ModuleSummation.cpp
@@ -116,9 +116,11 @@ void ModuleSummation::AddFPGAOutput(const DeviceOutput &input, const std::option
 }
 
 void ModuleSummation::AddEmptyOutput() {
+    std::unique_lock ul(module_summation_mutex);
     is_empty = true;
 }
 
 bool ModuleSummation::empty() const {
+    std::unique_lock ul(module_summation_mutex);
     return is_empty;
 }