Stop variable interpreted as target

In my makefile I accumulate option values in variable hopc.

Running the makefile as follows results in
make -f antares.mk split=3Dnode trlfn ndf  sm
make: *** No rule to make target 'trlfn'.  Stop.

How can I avoid the problem with trlfn and ndf?

hopc :=3D

ifneq ($(css),)
=09hopc +=3D --css-include=3D$(css)
endif

ifneq ($(css-ref),)
=09hopc +=3D --css-ref=3D$(css-ref)
endif

ifneq ($(links),)
=09hopc +=3D --internal-links=3D$(links)
endif

ifneq ($(split),)
=09hopc +=3D --split=3D$(split)
endif

ifneq ($(trlfn),)
=09hopc +=3D --transliterate-file-names
endif

ifneq ($(ndf),)
=09hopc +=3D --node-files
endif

$(hmldir)/antares.html: $(srcs) | $$(@D)
=09makeinfo $(opts) --html $(hopc) -o $@ $<