diff --git a/dlib/test/constexpr_if.cpp b/dlib/test/constexpr_if.cpp
index 9d48f06dc..97b51462d 100644
--- a/dlib/test/constexpr_if.cpp
+++ b/dlib/test/constexpr_if.cpp
@@ -4,6 +4,7 @@
 #include <string>
 #include <dlib/constexpr_if.h>
 #include <dlib/invoke.h>
+#include <cstdio>
 #include "tester.h"
 
 namespace
@@ -203,4 +204,4 @@ namespace
         }
     } a;
 
-}
\ No newline at end of file
+}