diff --git a/library/lua/argparse.lua b/library/lua/argparse.lua
index 8da11cb28..0a652f428 100644
--- a/library/lua/argparse.lua
+++ b/library/lua/argparse.lua
@@ -196,17 +196,16 @@ function coords(arg, arg_name, skip_validation)
     return pos
 end
 
+local toBool={["true"]=true,["yes"]=true,["y"]=true,["on"]=true,["1"]=true,
+              ["false"]=false,["no"]=false,["n"]=false,["off"]=false,["0"]=false}
 function boolean(arg, arg_name)
-    local toBool={["true"]=true,["yes"]=true,["y"]=true,["on"]=true,["1"]=true,
-                  ["false"]=false,["no"]=false,["n"]=false,["off"]=false,["0"]=false}
-
-    arg = string.lower(arg)
-    if toBool[arg] == nil then
+    local arg_lower = string.lower(arg)
+    if toBool[arg_lower] == nil then
         arg_error(arg_name,
-            'unknown value: "%s"; expected "true", "yes", "false", or "no"')
+            'unknown value: "%s"; expected "true", "yes", "false", or "no"', arg)
     end
 
-    return toBool[arg]
+    return toBool[arg_lower]
 end
 
 return _ENV