Your DAX actually looks spot on to me , and to confirm that I replicated your measure using the External Tools Practice Dataset, and it performs exactly as expected per the screenshot below:
Thus, I am left to the conclusion that the problem lies somewhere in your data model. If you can please provide a copy of your PBIX file, or at minimum a screenshot of your data model that will help us sort out exactly what the problem is and how to fix it.
Thanks.
– Brian