diff --git a/src/mp/test/foo.h b/src/mp/test/foo.h
index 65477c41..4dd9f376 100644
--- a/src/mp/test/foo.h
+++ b/src/mp/test/foo.h
@@ -7,6 +7,7 @@
 
 #include <map>
 #include <memory>
+#include <string>
 #include <vector>
 
 namespace mp {