LINQ GroupBy with conditional operator Error: Type of conditional expression cannot be determined

I have written below code but it throws an error

var conflictcomments = (item.Key == Constant.SB023) 
    ? item.GroupBy(m => new { m.UserGroup, m.IGGConflict }) 
    : item.GroupBy(m => new { m.CompanyCodeConflict, m.IGGConflict });

Error message:

Type of conditional expression cannot be determined because there is no implicit conversion between 'System.Collections.Generic.IEnumerable<System.Linq.IGrouping<<anonymous type: string UserGroup, string IGGConflict>, Entities.MitigatingControlComments>>' and 'System.Collections.Generic.IEnumerable<System.Linq.IGrouping<<anonymous type: string CompanyCodeConflict, string IGGConflict>, Entities.MitigatingControlComments>>'