diff --git a/src/FileContentsCache.hh b/src/FileContentsCache.hh
index 1d50be60..6f02af69 100644
--- a/src/FileContentsCache.hh
+++ b/src/FileContentsCache.hh
@@ -2,6 +2,7 @@
 
 #include <functional>
 #include <memory>
+#include <mutex>
 #include <shared_mutex>
 #include <string>
 #include <unordered_map>