uncaught TypeError: reduceInitial is not a function

I have an error that I cannot understand. It looks like I forgot to load a library but I've gone through all the available d3 libraries I can't see which library has reduceInitial as a function.

I use:

d3 v3.5.17

dc v2.0.5

crossfilter v1.3.14

jquery v3.6.1

d3-color v1.4.1

d3-format v1.4.5

d3-interpolate v1.4.0

d3-scale v2.2.2

d3-scale.chromatic v1.5.0

d3-time v1.1.0

d3 v2.3.0

Many thanks in advance for your help

This is the third parameter to group.reduce() and it expects a function.

You probably neglected to pass this parameter, so it's trying to call undefined as a function.