diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index 0b9a285..8fd856b 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -213,6 +213,9 @@
     MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len, mbedtls_x509_write_sig( &c2, buf,
                                         sig_oid, sig_oid_len, sig, sig_len ) );
 
+    if( len > (size_t)( c2 - buf ) )
+        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
     c2 -= len;
     memcpy( c2, c, len );