You need to add the default values of TEMPLATE_CONTEXT_PROCESSORS. However, instead of hard-coding those values, which will be tied to a specific version of Django, you can append your context processor to the default values by the following:
from django.conf import global_settings
TEMPLATE_CONTEXT_PROCESSORS = global_settings.TEMPLATE_CONTEXT_PROCESSORS + (
"myapp.processor.foos",
)
Make sure to include the trailing comma in the tuple, so that Python recognizes it as a tuple.