Error when querying interface

I am getting this error when prop3 is queried.

Either the Entity type should provide a \"resolveType\" function or each possible type should provide an \"isTypeOf\" function."

This is what my schema looks like (sample):

interface A { … }

interface B { … }

type C implements A {
prop1: value1
prop2: value2
prop3: [B]

I am using the grandstack starter project.

Can you provide your actual schema for this? Also the query you're running and the params? It might help a bit to see what you're actually sending, etc. Thank you.