diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index c3773ba..5120837 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -52,6 +52,10 @@
     TEST_ASSERT( olen >= pem_len - 1 );
     TEST_ASSERT( memcmp( buf, check_buf, pem_len - 1 ) == 0 );
 
+    ret = mbedtls_x509write_csr_der( &req, buf, pem_len / 2,
+                            rnd_pseudo_rand, &rnd_info );
+    TEST_ASSERT( ret == MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
 exit:
     mbedtls_x509write_csr_free( &req );
     mbedtls_pk_free( &key );
@@ -125,6 +129,10 @@
     TEST_ASSERT( olen >= pem_len - 1 );
     TEST_ASSERT( memcmp( buf, check_buf, pem_len - 1 ) == 0 );
 
+    ret = mbedtls_x509write_crt_der( &crt, buf, pem_len / 2,
+                            rnd_pseudo_rand, &rnd_info );
+    TEST_ASSERT( ret == MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
 exit:
     mbedtls_x509write_crt_free( &crt );
     mbedtls_pk_free( &issuer_key );