HEAD request and Content-Lenght

Hi, I encountered this funny issue while using proxygen as server.
On HEAD request I'm suppose to set Content-Lenght to value it would be and send no body.
So I do,
...
builder->header(proxygen::HTTP_HEADER_CONTENT_LENGTH, std::to_string(len));
builder->sendWithEOM();
however the latter sendWithEOM() (calls send()) resets the Content-Lenght back to zero.
I workarounded that by sending empty chunk first before sendWithEOM().

But I wonder if this is proxygen ResponseBuilder issue or I'm doing it wrong?