diff --git a/src/modules/Elsa.Workflows.Management/Filters/WorkflowInstanceFilter.cs b/src/modules/Elsa.Workflows.Management/Filters/WorkflowInstanceFilter.cs
index 27cc86ac2..6eeb37022 100644
--- a/src/modules/Elsa.Workflows.Management/Filters/WorkflowInstanceFilter.cs
+++ b/src/modules/Elsa.Workflows.Management/Filters/WorkflowInstanceFilter.cs
@@ -218,16 +218,16 @@ public class WorkflowInstanceFilter
         if (timestampFilters == null)
             yield break;
 
-        foreach (var timestampFilter in timestampFilters)
+        foreach (var (timestampFilter, index) in timestampFilters.Select((value, index) => (value, index)))
         {
             if (timestampFilter == null)
             {
-                yield return "Timestamp filter must be specified.";
+                yield return $"Timestamp filter at index {index} must be specified.";
                 continue;
             }
 
             if (!TryNormalizeTimestampFilterColumn(timestampFilter.Column, out _, out var error))
-                yield return error;
+                yield return $"Timestamp filter at index {index}: {error}";
         }
     }
 
@@ -251,7 +251,7 @@ public class WorkflowInstanceFilter
         if (normalizedColumn != null)
             return true;
 
-        error = $"Invalid timestamp filter column '{column}'. Allowed columns are: {string.Join(", ", TimestampFilterColumns)}.";
+        error = $"Invalid timestamp filter column. Allowed columns are: {string.Join(", ", TimestampFilterColumns)}.";
         return false;
     }
 
diff --git a/test/integration/Elsa.Alterations.IntegrationTests/AlterationsApiTimestampFilterTests.cs b/test/integration/Elsa.Alterations.IntegrationTests/AlterationsApiTimestampFilterTests.cs
index 372e866eb..f3b714770 100644
--- a/test/integration/Elsa.Alterations.IntegrationTests/AlterationsApiTimestampFilterTests.cs
+++ b/test/integration/Elsa.Alterations.IntegrationTests/AlterationsApiTimestampFilterTests.cs
@@ -97,7 +97,7 @@ public class AlterationsApiTimestampFilterTests : IAsyncLifetime
         var body = await response.Content.ReadAsStringAsync();
 
         Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
-        Assert.Contains("Timestamp filter must be specified.", body);
+        Assert.Contains("Timestamp filter at index 0 must be specified.", body);
     }
 
     [Theory]
@@ -109,8 +109,8 @@ public class AlterationsApiTimestampFilterTests : IAsyncLifetime
         var body = await response.Content.ReadAsStringAsync();
 
         Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
-        Assert.Contains("Timestamp filter column must be specified.", body);
-        Assert.Contains("Timestamp filter must be specified.", body);
+        Assert.Contains("Timestamp filter at index 0: Timestamp filter column must be specified.", body);
+        Assert.Contains("Timestamp filter at index 1 must be specified.", body);
     }
 
     private static object CreateRequest(string path)
diff --git a/test/integration/Elsa.Alterations.IntegrationTests/AssemblyInfo.cs b/test/integration/Elsa.Alterations.IntegrationTests/AssemblyInfo.cs
deleted file mode 100644
index 217120083..000000000
--- a/test/integration/Elsa.Alterations.IntegrationTests/AssemblyInfo.cs
+++ /dev/null
@@ -1,3 +0,0 @@
-using Xunit;
-
-[assembly: CollectionBehavior(DisableTestParallelization = true)]
diff --git a/test/unit/Elsa.Workflows.Management.UnitTests/Filters/WorkflowInstanceFilterTimestampTests.cs b/test/unit/Elsa.Workflows.Management.UnitTests/Filters/WorkflowInstanceFilterTimestampTests.cs
index ad3978d56..833731126 100644
--- a/test/unit/Elsa.Workflows.Management.UnitTests/Filters/WorkflowInstanceFilterTimestampTests.cs
+++ b/test/unit/Elsa.Workflows.Management.UnitTests/Filters/WorkflowInstanceFilterTimestampTests.cs
@@ -67,7 +67,9 @@ public class WorkflowInstanceFilterTimestampTests
         var exception = Assert.Throws<ArgumentException>(() => filter.Apply(_workflowInstances).ToList());
 
         Assert.Contains("Invalid timestamp filter column", exception.Message);
-        Assert.Contains("CreatedAt, UpdatedAt, FinishedAt", exception.Message);
+        Assert.DoesNotContain("CreatedAt == @0", exception.Message);
+        foreach (var column in WorkflowInstanceFilter.AllowedTimestampFilterColumns)
+            Assert.Contains(column, exception.Message);
         Assert.Equal($"{nameof(WorkflowInstanceFilter.TimestampFilters)}.Column", exception.ParamName);
     }
 
@@ -99,7 +101,7 @@ public class WorkflowInstanceFilterTimestampTests
         ]).ToList();
 
         var error = Assert.Single(errors);
-        Assert.Equal("Timestamp filter column must be specified.", error);
+        Assert.Equal("Timestamp filter at index 0: Timestamp filter column must be specified.", error);
     }
 
     [Fact]
@@ -108,7 +110,7 @@ public class WorkflowInstanceFilterTimestampTests
         var errors = WorkflowInstanceFilter.ValidateTimestampFilters([null!]).ToList();
 
         var error = Assert.Single(errors);
-        Assert.Equal("Timestamp filter must be specified.", error);
+        Assert.Equal("Timestamp filter at index 0 must be specified.", error);
     }
 
     private DateTimeOffset GetMatchingTimestamp(string column) => column switch