There are two ways to fix this.
1) Seems how IsValid is already a boolean, just take the value
<%= item.IsValid ? "Yes" : "No"%>
2) If you insist on converting it, compare it using Equals with the StringComparison.InvariantCultureIgnoreCase flag
<%= Convert.ToString(item.IsValid).Equals("True", StringComparison.InvariantCultureIgnoreCase) ? "Yes" : "No"%>