diff --git a/libcxx/test/support/format.functions.common.h b/libcxx/test/support/format.functions.common.h
index 769b1aff9c642..81ed04998a510 100644
--- a/libcxx/test/support/format.functions.common.h
+++ b/libcxx/test/support/format.functions.common.h
@@ -11,6 +11,7 @@
 // Contains the common part of the formatter tests for different papers.
 
 #include <algorithm>
+#include <cctype>
 #include <charconv>
 #include <format>
 #include <ranges>