diff --git a/api/include/opentelemetry/baggage/baggage.h b/api/include/opentelemetry/baggage/baggage.h
index f2a2a4fbdc..7f904f09a7 100644
--- a/api/include/opentelemetry/baggage/baggage.h
+++ b/api/include/opentelemetry/baggage/baggage.h
@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <cctype>
+
 #include "opentelemetry/common/kv_properties.h"
 #include "opentelemetry/nostd/shared_ptr.h"
 #include "opentelemetry/nostd/string_view.h"