Fine grained access control with cognito identity and custom claims

Question

I have a cognito identity pool that assigns a principal tag named home. I create credentials with fromCognitoIdentityPool (which ultimately calls AssumeRoleWithIdentity ).

Further, I use these credentials to invoke a lambda named locate which has the following policy for allowing access to only items with primary key same as the value of home:

        - Statement:
            - Action: [dynamodb:GetItem]
              Effect: Allow
              Resource: !Sub "arn:${AWS::Partition}:dynamodb:${AWS::Region}:${AWS::AccountId}:table/${PlacesTableName}"
              Condition:
                ForAllValues:StringEquals:
                  dynamodb:LeadingKeys:
                    - "${aws:PrincipalTag/home}"

However, this condition always fails and I get access denied exception:

AccessDeniedException: User: arn:aws:sts::...:assumed-role/...-UserRole-.../Locate is not authorized to perform: dynamodb:GetItem on resource: arn:aws:dynamodb:...:table/Places because no identity-based policy allows the dynamodb:GetItem action

Any ideas?