diff --git a/src/FileContentsCache.hh b/src/FileContentsCache.hh
index 10605a98..65245c18 100644
--- a/src/FileContentsCache.hh
+++ b/src/FileContentsCache.hh
@@ -3,6 +3,7 @@
 #include <memory>
 #include <string>
 #include <unordered_map>
+#include <functional>
 
 using namespace std;