diff --git a/missions/strings.lua b/missions/strings.lua
index fde3289..2ca988d 100644
--- a/missions/strings.lua
+++ b/missions/strings.lua
@@ -122,7 +122,7 @@ end
 
 function test_string_upper()
   local str = "eat me"
-  assert_equal(__, string.upper('eat me'))
+  assert_equal(__, string.upper(str))
 end
 
 function test_string_byte_returns_the_first_ascii_char()