diff --git a/src/include/ipxe/x509.h b/src/include/ipxe/x509.h
index 37c5a4ee..2ea21aa9 100644
--- a/src/include/ipxe/x509.h
+++ b/src/include/ipxe/x509.h
@@ -10,6 +10,7 @@
 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <stdint.h>
+#include <stdlib.h>
 
 struct asn1_cursor;