diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index 9e33910751..35bae2429f 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -1605,6 +1605,10 @@ server2-sha256.crt: server2.req.sha256
 	$(MBEDTLS_CERT_WRITE) request_file=server2.req.sha256 serial=2 issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) not_before=20190210144406 not_after=20290210144406 md=SHA256 version=3 output_file=$@
 all_final += server2-sha256.crt
 
+server2-badsign.crt: server2.crt
+	{ head -n-2 $<; tail -n-2 $< | sed -e '1s/0\(=*\)$$/_\1/' -e '1s/[^_=]\(=*\)$$/0\1/' -e '1s/_/1/'; } > $@
+all_final += server2-badsign.crt
+
 # server3*
 
 parse_input/server3.crt server3.crt: server3.key