diff --git a/userspace/engine/formats.cpp b/userspace/engine/formats.cpp
index d03801b4..ae7571e7 100644
--- a/userspace/engine/formats.cpp
+++ b/userspace/engine/formats.cpp
@@ -269,8 +269,8 @@ int falco_formats::format_event (lua_State *ls)
 
 int falco_formats::resolve_tokens(lua_State *ls)
 {
-	if (!lua_isstring(ls, -1) ||
-	    !lua_isstring(ls, -2)) {
+	if (!lua_isuserdata(ls, 1) ||
+	    !lua_isstring(ls, 2)) {
 		lua_pushstring(ls, "Invalid arguments passed to resolve_tokens()");
 		lua_error(ls);
 	}
diff --git a/userspace/engine/formats.h b/userspace/engine/formats.h
index 955737ac..01677a63 100644
--- a/userspace/engine/formats.h
+++ b/userspace/engine/formats.h
@@ -53,6 +53,7 @@ class falco_formats
 	// formatted_string = falco.format_event(evt, formatter)
 	static int format_event(lua_State *ls);
 
+	// resolve_tokens = falco.resolve_tokens(evt, formatter)
 	static int resolve_tokens(lua_State *ls);
 
 	static sinsp* s_inspector;