Null coalescing operator sometimes doesn't work

Using the C# Editor sample in the .Net Languages Add-on Demos of the Sample Browser app. If I use the following code snippet the line with '(a ?? "").Trim();' gets picked up as an error. However wrapping the null coalescing statement in another parenthesis group clears the error.

string a = null;
Console.WriteLine((a ?? "").ToString());
(a ?? "").Trim();
((a ?? "")).Trim();

The code snippet is redundant, but highlights the issue.