diff --git a/common/json.c b/common/json.c
index 67cc1e60e..2dcd07fb2 100644
--- a/common/json.c
+++ b/common/json.c
@@ -784,13 +784,19 @@ void json_add_literal(struct json_stream *result, const char *fieldname,
 		memcpy(dest, literal, strlen(literal));
 }
 
-void json_add_string(struct json_stream *result, const char *fieldname, const char *value TAKES)
+void json_add_stringn(struct json_stream *result, const char *fieldname,
+		      const char *value TAKES, size_t value_len)
 {
-	json_add_member(result, fieldname, true, "%s", value);
+	json_add_member(result, fieldname, true, "%.*s", (int)value_len, value);
 	if (taken(value))
 		tal_free(value);
 }
 
+void json_add_string(struct json_stream *result, const char *fieldname, const char *value TAKES)
+{
+	json_add_stringn(result, fieldname, value, strlen(value));
+}
+
 void json_add_bool(struct json_stream *result, const char *fieldname, bool value)
 {
 	json_add_member(result, fieldname, false, value ? "true" : "false");
diff --git a/common/json.h b/common/json.h
index 19ce012a9..c00e5829a 100644
--- a/common/json.h
+++ b/common/json.h
@@ -163,6 +163,13 @@ const jsmntok_t *json_delve(const char *buffer,
  */
 void json_add_string(struct json_stream *result, const char *fieldname, const char *value);
 
+/* '"fieldname" : "value[:value_len]"' or '"value[:value_len]"' if
+ * fieldname is NULL.  Turns any non-printable chars into JSON
+ * escapes, but leaves existing escapes alone.
+ */
+void json_add_stringn(struct json_stream *result, const char *fieldname,
+		      const char *value TAKES, size_t value_len);
+
 /* '"fieldname" : "value"' or '"value"' if fieldname is NULL.  String must
  * already be JSON escaped as necessary. */
 void json_add_escaped_string(struct json_stream *result,