Can't cast database type char to String

With target framework netcoreapp3.1, using a PostgreSQL database with NpgSQL Nuget Package (v. 3.1.4), I have a column my_column with type char containing numbers and characters, e.g.:

> SELECT DISTINCT my_column FROM my_table;
  my_column 
-----------------------
 2
 1
 0
 d

When trying to access the properties with the Entity Framework :

modelBuilder.Entity<MyModel>(entity =>
{
  entity.Property(e => e.MyColumn)
   .HasColumnName("my_column")
   .HasColumnType("char");
}

With MyModel like:

public partial class MyModel
{
     public string MyColumn { get; set; }
}

I get this error (when requesting the data from my controller):

System.InvalidCastException: Can't cast database type char to String
   at Npgsql.NpgsqlDataReader.GetFieldValue[T](Int32 ordinal)
   at Npgsql.NpgsqlDataReader.GetString(Int32 ordinal)
   at lambda_method(Closure , QueryContext , DbDataReader , ResultContext , Int32[] , ResultCoordinator )
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Controllers.Controller.Get() in ~/Controllers/Controller.cs:line 31
   at lambda_method(Closure , Object )
   at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

Does this have to do with some char-values in my table are numbers? Or are they not casted correctly somewhere else?