Presumably you're applying this expression to the Source Text property of a text layer? Try this: thisComp.layer("Null 2").effect("Slider Control")("Slider").value + "mm"; It works completly, I put an string linked to a slider on the source text parameter on ...

That's it, I'm looking for an string to mix numeric data (from an slider) with letters (messuarement), in this way: thisComp.layer("Null 2").effect("Slider Control")("Slider") + mm to get: ## mm I'm getting error puting the above string, and I think is because I'm mixin...