diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index c90f8bc..c8169f1 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -474,7 +474,7 @@
     }
 
     if( ( ret = mbedtls_pk_sign( ctx->issuer_key, ctx->md_alg,
-                                 hash, 0, sig, &sig_len,
+                                 hash, 0, sig, sizeof( sig ), &sig_len,
                                  f_rng, p_rng ) ) != 0 )
     {
         return( ret );