"KeyError: 'MONGO_SSL'" build error in non-SSL legacy builds

When I use "scons build-unit" or "scons unit" I get following error and build fails:

"KeyError: 'MONGO_SSL':
File "/root/mongo-cxx-driver/SConstruct", line 1907:
env.SConscript('src/SConscript', variant_dir='$VARIANT_DIR', duplicate=False)
File "/usr/lib/scons/SCons/Script/SConscript.py", line 551:
return _SConscript(self.fs, *files, **subst_kw)
File "/usr/lib/scons/SCons/Script/SConscript.py", line 260:
exec file in call_stack[-1].globals
File "/root/mongo-cxx-driver/src/SConscript", line 8:
env.SConscript('SConscript.client')
File "/usr/lib/scons/SCons/Script/SConscript.py", line 551:
return _SConscript(self.fs, *files, **subst_kw)
File "/usr/lib/scons/SCons/Script/SConscript.py", line 260:
exec file in call_stack[-1].globals
File "/root/mongo-cxx-driver/src/SConscript.client", line 180:
usingSsl = libEnv['MONGO_SSL']
File "/usr/lib/scons/SCons/Environment.py", line 410:
return self._dict[key]
"