diff --git a/src/test/basics/Number_test.cpp b/src/test/basics/Number_test.cpp
index 7d04022a19..26060c70e9 100644
--- a/src/test/basics/Number_test.cpp
+++ b/src/test/basics/Number_test.cpp
@@ -9,6 +9,7 @@
 #include <boost/multiprecision/number.hpp>
 
 #include <array>
+#include <cctype>
 #include <cstdint>
 #include <limits>
 #include <map>