diff --git a/include/cpr/callback.h b/include/cpr/callback.h
index aec274106..4b470513a 100644
--- a/include/cpr/callback.h
+++ b/include/cpr/callback.h
@@ -5,6 +5,7 @@
 
 #include <atomic>
 #include <functional>
+#include <cstdint>
 #include <optional>
 #include <utility>