diff --git a/library/hmac_drbg.c b/library/hmac_drbg.c
index 2ebdac0..2b79ee5 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -34,6 +34,7 @@
 #if defined(MBEDTLS_HMAC_DRBG_C)
 
 #include "mbedtls/hmac_drbg.h"
+#include "mbedtls/platform.h"
 #include "mbedtls/platform_util.h"
 
 #include <string.h>