Problem installing HPLIP 3.12.6 on Ubuntu 18.042 LTS

Hello!
I am trying to install HPLIP to drive my new printer, HP OfficeJet Pro 7720.
I followed the instructions here: https://developers.hp.com/hp-linux-imaging-and-printing/install/manual/distros/ubuntu
All goes well until Step 4.
When I type make, I get the error message:
prnt/cupsext/cupsext.c: In function 'getPrinters':
prnt/cupsext/cupsext.c:336:12: error: dereferencing pointer to incomplete type 'ipp_t {aka struct _ipp_s}'
     request->request.op.operation_id = CUPS_GET_PRINTERS;
            ^~
prnt/cupsext/cupsext.c:381:64: error: dereferencing pointer to incomplete type 'ipp_attribute_t {aka struct _ipp_attribute_s}'
         for ( attr = response->attrs; attr != NULL; attr = attr->next )
                                                                ^~
Makefile:5678: recipe for target 'cupsext_la-cupsext.lo' failed
make: *** [cupsext_la-cupsext.lo] Error 1

Any idea how I can correct/circumvent this?
Thanks in advance,
Alex