diff --git a/include/spdlog/common-inl.h b/include/spdlog/common-inl.h
index 77064756..4ce5e91d 100644
--- a/include/spdlog/common-inl.h
+++ b/include/spdlog/common-inl.h
@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <iterator>
+#include <cctype>
 
 namespace spdlog {
 namespace level {