diff --git a/src/logic/scripting/lua/libs/libutf8.cpp b/src/logic/scripting/lua/libs/libutf8.cpp
index 15fc54e92..163f07e4e 100644
--- a/src/logic/scripting/lua/libs/libutf8.cpp
+++ b/src/logic/scripting/lua/libs/libutf8.cpp
@@ -57,7 +57,12 @@ static int l_sub(lua::State* L) {
     if (lua::gettop(L) >= 3) {
         end = std::max(0, static_cast<int>(lua::tointeger(L, 3) - 1));
     }
-    return lua::pushstring(L, util::u32str2str_utf8(string.substr(start, end)));
+    return lua::pushstring(
+        L,
+        util::u32str2str_utf8(
+            string.substr(start, (end >= start ? end - start + 1 : 0))
+        )
+    );
 }
 
 static int l_upper(lua::State* L) {