# T102

> operator not defined for these operand types

**Severity** Error

The operator is applied to operand types it does not support, for example adding two colors, multiplying a string, or comparing a bool with `<`. Arithmetic works on `int` and `float`, `+` also joins two strings, and time values combine with durations.

Use the dedicated function for the job, such as `color.mix` for blending or `str.format` for text, or convert the operands first.

## Failing example

```flowscope
script "Blended line"
data chart = subscribe(data.ohlcv)
plot mid = plot.line(title: "Midpoint")
on chart.close {
  let tint = color.bull + color.bear
  mid.plot((chart.high + chart.low) / 2.0, color: tint)
}
```

## Fixed example

```flowscope
script "Blended line"
data chart = subscribe(data.ohlcv)
plot mid = plot.line(title: "Midpoint")
on chart.close {
  let tint = color.mix(color.bull, color.bear, 0.5)
  mid.plot((chart.high + chart.low) / 2.0, color: tint)
}
```

## Related

- [T101](/docs/scripting/reference/diagnostics/t101)