diff --git a/reg-tests/converter/json.vtc b/reg-tests/converter/json.vtc
new file mode 100644
index 0000000000..efac8f6226
--- /dev/null
+++ b/reg-tests/converter/json.vtc
@@ -0,0 +1,41 @@
+varnishtest "json converter test"
+
+#REQUIRE_VERSION=1.6
+
+feature ignore_unknown_macro
+
+server s1 {
+	rxreq
+	txresp
+} -repeat 2 -start
+
+haproxy h1 -conf {
+    defaults
+	mode http
+	timeout connect 1s
+	timeout client  1s
+	timeout server  1s
+
+    frontend fe
+	bind "fd@${fe}"
+
+	http-response set-header json0 "%[str(foo 1/2),json]"
+	# bad UTF-8 sequence
+	http-response set-header json1 "%[str(\xE0),json(utf8)]"
+	# bad UTF-8 sequence, but removes them
+	http-response set-header json2 "%[str(-\xE0-),json(utf8s)]"
+
+	default_backend be
+
+    backend be
+	server s1 ${s1_addr}:${s1_port}
+} -start
+
+client c1 -connect ${h1_fe_sock} {
+	txreq -url "/"
+	rxresp
+	expect resp.http.json0 == "foo 1\\/2"
+	expect resp.http.json1 == ""
+	expect resp.http.json2 == "--"
+	expect resp.status == 200
+} -run
diff --git a/src/sample.c b/src/sample.c
index 41d95d86c2..fa854632f4 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -2152,21 +2152,12 @@ enum input_type {
 	IT_UTF8P,
 	IT_UTF8PS,
 };
+
 static int sample_conv_json_check(struct arg *arg, struct sample_conv *conv,
                                   const char *file, int line, char **err)
 {
 	enum input_type type;
 
-	if (!arg) {
-		memprintf(err, "Unexpected empty arg list");
-		return 0;
-	}
-
-	if (arg->type != ARGT_STR) {
-		memprintf(err, "Unexpected arg type");
-		return 0;
-	}
-
 	if (strcmp(arg->data.str.area, "") == 0)
 		type = IT_ASCII;
 	else if (strcmp(arg->data.str.area, "ascii") == 0)