diff --git a/src/cpp/MultiSketchIndex.h b/src/cpp/MultiSketchIndex.h
index f0d7461..07130d4 100644
--- a/src/cpp/MultiSketchIndex.h
+++ b/src/cpp/MultiSketchIndex.h
@@ -3,6 +3,7 @@
 
 #include <iostream>
 #include <vector>
+#include <unordered_map>
 
 
 #ifndef HASH_T