diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 80913ec..23d46ee 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -408,6 +408,9 @@
     ASN1_CHK_ADD( sig_and_oid_len, x509_write_sig( &c2, buf,
                                         sig_oid, sig_oid_len, sig, sig_len ) );
 
+    if( len > (size_t)( c2 - buf ) )
+        return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
+
     c2 -= len;
     memcpy( c2, c, len );