Add role to another role trust relationship via nodejs

Question

I want to add a role to another role trust relationship via nodejs aws library. I have tried to create a policy and than add it to the main role but its not working what i'm doing wrong

 createTrustPolicy(RoleName) {
        return new Promise((resolve, reject) => {
            const params = {
                PolicyDocument: '{' +
                    '  "Version": "2012-10-17",' +
                    '  "Statement": [' +
                    '    {' +
                    '      "Effect": "Allow",' +
                    `      "Principal": {"AWS":"arn:aws:iam::${aws_account_id}:role/${RoleName}"},` +
                    '      "Action": "sts:AssumeRole"' +
                    '    }' +
                    '  ]' +
                    '}',
                PolicyName: `TrustRelationship_${RoleName}`,
                Description: 'The Policy that will be used to add the green grass role to be added to trust relationship',

            };
            this.iam.createPolicy(params, (err, data) =>
                (err && err.name !== 'EntityAlreadyExists') ? reject(err) : resolve(data));
        })
    }

    /**
     * Attach the Trust Policy to the aws
     * role who is federated to use gcp identity.
     * @param {*} RoleName the Greengrass Role Name
     */

    AttachTrustPolicyToFederatedRole(RoleName) {
        return new Promise((resolve, reject) => {
            const params = {
                PolicyArn: `arn:aws:iam::${aws_account_id}:policy/TrustRelationship_${RoleName}`,
                RoleName: "federation_pubsub_greengrass_to_itclient"
            }
            this.iam.attachRolePolicy(params, (err, data) =>
                (err ? reject(err) : resolve(data)));
        })
    }

and i get the following error :

MalformedPolicyDocument: Policy document should not specify a principal..