diff --git a/include/openssl/span.h b/include/openssl/span.h
index 1d732eb..7410bf9 100644
--- a/include/openssl/span.h
+++ b/include/openssl/span.h
@@ -21,8 +21,9 @@
 
 extern "C++" {
 
+#include <stdlib.h>
+
 #include <algorithm>
-#include <cstdlib>
 #include <type_traits>
 
 BSSL_NAMESPACE_BEGIN