diff --git a/YD_AllHeartRates.Api/Services/Impl/HeartRateReportService.cs b/YD_AllHeartRates.Api/Services/Impl/HeartRateReportService.cs
index c1b2ca4..8815e09 100644
--- a/YD_AllHeartRates.Api/Services/Impl/HeartRateReportService.cs
+++ b/YD_AllHeartRates.Api/Services/Impl/HeartRateReportService.cs
@@ -140,7 +140,7 @@ namespace YD_AllHeartRates.Api.Services.Impl
                     ClassId = c.Id,
                     ClassName = c.ClassName,
                     GradeName = c.GradeName,
-                    TeacherName = t != null ? t.TeacherName : null
+                    TeacherName = t != null ? t.TeacherName : ""
                 })
                 .OrderBy(x => x.ClassId)
                 .ToListAsync();