Why cant i have guid primary keys entity framework :(

gistfile1.cs

     if (entitySet.ElementType.KeyMembers.Count > 0)
        {
            builder.Append("    PRIMARY KEY (");
            bool flag = true;
            foreach (EdmMember member in entitySet.ElementType.KeyMembers)
            {
                if (IsServerGeneratedGuid(member))
                {
                    throw ADP1.ServerGeneratedGuidKeyNotSupportedException(member.Name);
                }
                if (flag)
                {
                    flag = false;
                }
                else
                {
                    builder.Append(",");
                }
                AppendIdentifier(builder, member.Name);
            }
            builder.Append(")");
            builder.AppendLine();
        }