diff --git a/libs/core/format/include/hpx/util/from_string.hpp b/libs/core/format/include/hpx/util/from_string.hpp
index 07eb9b68dfb..c3d0d857143 100644
--- a/libs/core/format/include/hpx/util/from_string.hpp
+++ b/libs/core/format/include/hpx/util/from_string.hpp
@@ -10,6 +10,7 @@
 #include <hpx/util/bad_lexical_cast.hpp>
 
 #include <algorithm>
+#include <cctype>
 #include <cstddef>
 #include <limits>
 #include <sstream>