Possible 'null' assignment to entity with '[NotNull]' attribute even when null check is explicitly specified (dynamic type)

Consider the following code (this is not production code, just used for demonstration):

Snippet

private void Test()
{
    dynamic selection = ExcelApp.Selection;
    if (selection != null && Marshal.IsComObject(selection))
        Marshal.ReleaseComObject(selection);
}

The selection variable in the last line is flagged for possible null.

I assume the problem is because the variable is a dynamic one. Nonetheless I can't see why R# complains about it.

(R# 2017.1.1, VS2017 Enterprise 15.1 26403.7)