diff --git a/programs/pkey/pk_sign.c b/programs/pkey/pk_sign.c
index 47a098a..104e49f 100644
--- a/programs/pkey/pk_sign.c
+++ b/programs/pkey/pk_sign.c
@@ -56,6 +56,7 @@
 #include "mbedtls/ctr_drbg.h"
 #include "mbedtls/md.h"
 #include "mbedtls/pk.h"
+#include "mbedtls/bignum.h"
 
 #include <stdio.h>
 #include <string.h>