attempt to concatenate a nil value

The problem

The console shows «attempt to concatenate a nil value (local 'x')» and the script stops on that line.

The cause

You're joining with `..` a variable that is `nil` (data that didn't arrive: an empty DB field, an argument that wasn't passed, a getter that returned nil). Lua can't concatenate nil with text.

The solution

Check the value before concatenating and give it a default:

-- Instead of:
local msg = 'Hello ' .. name        -- name may be nil -> error

-- Protect with a default:
local msg = 'Hello ' .. (name or 'unknown')

-- Or validate first:
if not name then return end
local msg = 'Hello ' .. name

-- For numbers, convert them: tostring(value)
print('Balance: ' .. tostring(amount))

Step by step

1. 1.Look at the name in parentheses in the error (local 'x' / field 'y'): that's the nil variable.
2. 2.Trace where it comes from: an empty MySQL query? an event argument that didn't arrive?
3. 3.Protect with `(value or 'default')` or validate with `if not value then return end`.
4. 4.To concatenate numbers use `tostring(n)`.
5. 5.If it comes from the DB, make sure the row exists before using its fields.

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