diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index cd3be09..6983d60 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -32,6 +32,7 @@
 
 #include "x509.h"
 #include "x509_crl.h"
+#include "bignum.h"
 
 /**
  * \addtogroup x509_module