diff --git a/library/asn1write.c b/library/asn1write.c
index 6e5badb..0ec1647 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -33,6 +33,7 @@
 
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
+#include "mbedtls/platform_util.h"
 #else
 #include <stdlib.h>
 #define mbedtls_calloc    calloc