конечный вариант

AsbCloudInfrastructure/Services/Subsystems/SubsystemOperationTimeService.cs

@@ -206,7 +206,7 @@ namespace AsbCloudInfrastructure.Services.Subsystems
             var result =activeWell.Select( t => new SubsystemActiveWellStatDto
                         {
                             ActiveWell = t
-                        });
+                        }).ToList();
 
             var query = db.SubsystemOperationTimes
                 .Where(o => telemetryIds.Contains(o.IdTelemetry))