parse error on if statement with negated variable

[CuriousRichard]

It seems like negated if statements don’t work like I expected. I have

{%- if not variablename -%} thenclause {%- else -%} elseclause {%- endif -%}

which I believe to be valid Jinja2, but I get a parse error at the first character of my variable.
It appears that this is fixable with parenthesis around the variable name, or removing the not and
switching the clauses. Is there a way to make Ginger accept this without changes?.