# M0016 invalid operation (/docs/diagnostics/codes/m0016)

<Diagnostic code="M0016" />

## The mistake and the fix [#the-mistake-and-the-fix]

<div className="code-pair">

```go title="The mistake"
var m, n map[string]int
_ = m < n // maps have no order
```

```go title="Fixed"
_ = len(m) < len(n) // [!code highlight]
```

</div>

See also [all error codes](/docs/diagnostics/codes) and [marmot explain](/docs/commands/explain).