diff --git a/src/MSDIAL5/MsdialCore/Parameter/ParameterBase.cs b/src/MSDIAL5/MsdialCore/Parameter/ParameterBase.cs
index ec3186eef..895fdd811 100644
--- a/src/MSDIAL5/MsdialCore/Parameter/ParameterBase.cs
+++ b/src/MSDIAL5/MsdialCore/Parameter/ParameterBase.cs
@@ -1221,8 +1221,25 @@ public class AlignmentBaseParameter {
         public bool TogetherWithAlignment { get; set; } = true;
         [Key(9)]
         public bool UseRefMatchedPeaksOnly { get; set; } = false;
+        /// <remarks>
+        /// Key 10 was added after projects with keys 0-9 were saved. Such a project has no element here, and MessagePack
+        /// then assigns null rather than the initializer, so the getter hands out the defaults instead of null: an older
+        /// project opens, exports its parameters and aligns with automatic RT correction off, as it was saved. The
+        /// defaults are put in place once, atomically, as for <see cref="ParameterBase.LipidRtCheckParam"/>.
+        /// </remarks>
         [Key(10)]
-        public AutomaticAlignmentRetentionTimeCorrectionParameter AutomaticRtCorrection { get; set; } = new AutomaticAlignmentRetentionTimeCorrectionParameter();
+        public AutomaticAlignmentRetentionTimeCorrectionParameter AutomaticRtCorrection {
+            get {
+                var parameter = _automaticRtCorrection;
+                if (parameter is null) {
+                    System.Threading.Interlocked.CompareExchange(ref _automaticRtCorrection, new AutomaticAlignmentRetentionTimeCorrectionParameter(), null);
+                    parameter = _automaticRtCorrection;
+                }
+                return parameter;
+            }
+            set => _automaticRtCorrection = value;
+        }
+        private AutomaticAlignmentRetentionTimeCorrectionParameter _automaticRtCorrection = new AutomaticAlignmentRetentionTimeCorrectionParameter();
     }
 
     /// <summary>
diff --git a/tests/MSDIAL5/MsdialCoreTests/Parameter/AlignmentBaseParameterTests.cs b/tests/MSDIAL5/MsdialCoreTests/Parameter/AlignmentBaseParameterTests.cs
new file mode 100644
index 000000000..253fbf461
--- /dev/null
+++ b/tests/MSDIAL5/MsdialCoreTests/Parameter/AlignmentBaseParameterTests.cs
@@ -0,0 +1,85 @@
+using CompMs.Common.MessagePack;
+using MessagePack;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System.IO;
+using System.Linq;
+
+namespace CompMs.MsdialCore.Parameter.Tests
+{
+    /// <summary>
+    /// AutomaticRtCorrection is MessagePack key 10, added after projects with keys 0-9 were saved. MessagePack 1.x
+    /// gives an absent key its type's default, not the member initialiser, so the nested parameter loaded as null and
+    /// the parameter export (and the GUI's alignment view) threw on an older project.
+    /// </summary>
+    [TestClass()]
+    public class AlignmentBaseParameterTests
+    {
+        [TestMethod()]
+        public void AProjectSavedBeforeAutomaticRtCorrectionExistedLoadsWithItsDefaults() {
+            var saved = new ParameterWithoutAutomaticRtCorrection { AlignmentReferenceFileID = 3, RetentionTimeAlignmentTolerance = 0.2F, TogetherWithAlignment = false, };
+
+            var loaded = SaveAndLoadAsProjectsDo<ParameterWithoutAutomaticRtCorrection, AlignmentBaseParameter>(saved);
+
+            Assert.IsNotNull(loaded.AutomaticRtCorrection);
+            Assert.IsFalse(loaded.AutomaticRtCorrection.Execute, "an older project aligns without automatic RT correction, as it was saved");
+            Assert.AreEqual(new AutomaticAlignmentRetentionTimeCorrectionParameter().LocalSupportRtWindow, loaded.AutomaticRtCorrection.LocalSupportRtWindow);
+            Assert.AreSame(loaded.AutomaticRtCorrection, loaded.AutomaticRtCorrection, "the defaults are created once and kept");
+            Assert.AreEqual(3, loaded.AlignmentReferenceFileID);
+            Assert.AreEqual(0.2F, loaded.RetentionTimeAlignmentTolerance);
+            Assert.IsFalse(loaded.TogetherWithAlignment);
+        }
+
+        [TestMethod()]
+        public void AnOlderProjectExportsItsParameters() {
+            var loaded = SaveAndLoadAsProjectsDo<ParameterWithoutAutomaticRtCorrection, AlignmentBaseParameter>(new ParameterWithoutAutomaticRtCorrection());
+            var parameter = new ParameterBase { AlignmentBaseParam = loaded, };
+
+            var text = parameter.ParametersAsText();
+
+            Assert.IsTrue(text.Contains("Execute automatic RT correction for alignment: False"));
+        }
+
+        [TestMethod()]
+        public void AStoredAutomaticRtCorrectionIsKept() {
+            var saved = new AlignmentBaseParameter();
+            saved.AutomaticRtCorrection.Execute = true;
+            saved.AutomaticRtCorrection.ReferenceFileId = 5;
+
+            var loaded = SaveAndLoadAsProjectsDo<AlignmentBaseParameter, AlignmentBaseParameter>(saved);
+
+            Assert.IsTrue(loaded.AutomaticRtCorrection.Execute);
+            Assert.AreEqual(5, loaded.AutomaticRtCorrection.ReferenceFileId);
+        }
+
+        [TestMethod()]
+        public void SettingNullHandsOutTheDefaults() {
+            var parameter = new AlignmentBaseParameter { AutomaticRtCorrection = null, };
+
+            Assert.IsNotNull(parameter.AutomaticRtCorrection);
+            Assert.IsFalse(parameter.AutomaticRtCorrection.Execute);
+        }
+
+        private static TLoad SaveAndLoadAsProjectsDo<TSave, TLoad>(TSave saved) {
+            using var stream = new MemoryStream();
+            MessagePackDefaultHandler.SaveToStream(saved, stream);
+            stream.Position = 0;
+            return MessagePackDefaultHandler.LoadFromStream<TLoad>(stream);
+        }
+
+        /// <summary>The parameter as it was serialised before key 10 (AutomaticRtCorrection) existed.</summary>
+        [MessagePackObject]
+        public sealed class ParameterWithoutAutomaticRtCorrection
+        {
+            [Key(0)] public int AlignmentReferenceFileID { get; set; } = 0;
+            [Key(1)] public float RetentionTimeAlignmentFactor { get; set; } = 0.5F;
+            [Key(2)] public float Ms1AlignmentFactor { get; set; } = 0.5F;
+            [Key(3)] public float SpectrumSimilarityAlignmentFactor { get; set; } = 0.5F;
+            [Key(4)] public float Ms1AlignmentTolerance { get; set; } = 0.015F;
+            [Key(5)] public float RetentionTimeAlignmentTolerance { get; set; } = 0.10F;
+            [Key(6)] public float SpectrumSimilarityAlignmentTolerance { get; set; } = 0.8F;
+            [Key(7)] public float AlignmentScoreCutOff { get; set; } = 50;
+            [Key(8)] public bool TogetherWithAlignment { get; set; } = true;
+            [Key(9)] public bool UseRefMatchedPeaksOnly { get; set; } = false;
+        }
+    }
+}