How to customize the default error message "The value '' is invalid"

Question

I am new to asp.net core. I have a form which is generated dynamically. I want the validation error message to be "The value must be numeric" instead I get

"The value 'a' is invalid."

Here is my View Model:

[RegularExpression("^[0-9]*$", ErrorMessage = "The value must be numeric")]
public List<int> Units_Service { get; set; }

Here is my form code:

for (int i = 0; i < Model.Workload_Category_Name.Count; i++)
{
   <div class="row">
       <div class="col-md-3">
           <b><u>  @Html.DisplayFor(model => model.Workload_Category_Name[i])</u> </b>
       </div>
       <div class="col-md-4">
          @Html.TextBoxFor(model => model.Units_Service[i], new { style = "width: 15%", MaskedTextBox = "9999" })
          @Html.ValidationMessageFor(model => model.Units_Service[i])
       </div>
   </div>
 }

Despite the fact, I have put the custom error message in my View Model as shown above, I keep getting the default message "The value '' is invalid". Please what is the solution to this kind of scenario ?