diff --git a/src/DiffEngineTray.Tests/OptionsFormMaxInstancesTests.cs b/src/DiffEngineTray.Tests/OptionsFormMaxInstancesTests.cs
new file mode 100644
index 00000000..2e257467
--- /dev/null
+++ b/src/DiffEngineTray.Tests/OptionsFormMaxInstancesTests.cs
@@ -0,0 +1,27 @@
+using NumericUpDown = System.Windows.Forms.NumericUpDown;
+
+/// <summary>
+/// The form's range has to be the setting's range. DiffEngine_MaxInstances takes any ushort, and
+/// the spinner was left at NumericUpDown's default maximum of 100, so anything above that threw
+/// out of the constructor and Options could never be opened to lower it again.
+/// </summary>
+[TUnit.Core.Executors.STAThreadExecutor]
+public class OptionsFormMaxInstancesTests
+{
+    [Test]
+    public async Task Opens_at_the_highest_max_instances_that_can_be_set()
+    {
+        using var form = new OptionsForm(
+            new()
+            {
+                MaxInstancesToLaunch = ushort.MaxValue
+            },
+            _ => Task.FromResult<IReadOnlyCollection<string>>([]));
+
+        var spinner = form.Controls
+            .Find("maxInstancesNumericUpDown", true)
+            .OfType<NumericUpDown>()
+            .Single();
+        await Assert.That(spinner.Value).IsEqualTo(ushort.MaxValue);
+    }
+}
diff --git a/src/DiffEngineTray/Settings/OptionsForm.Designer.cs b/src/DiffEngineTray/Settings/OptionsForm.Designer.cs
index f4e13760..7bee9b0f 100644
--- a/src/DiffEngineTray/Settings/OptionsForm.Designer.cs
+++ b/src/DiffEngineTray/Settings/OptionsForm.Designer.cs
@@ -256,6 +256,16 @@ void InitializeComponent()
         this.maxInstancesNumericUpDown.Dock = System.Windows.Forms.DockStyle.Left;
         this.maxInstancesNumericUpDown.Location = new System.Drawing.Point(5, 44);
         this.maxInstancesNumericUpDown.Margin = new System.Windows.Forms.Padding(5);
+        // ushort.MaxValue, which is what MaxInstance accepts. Left at the default 100, a
+        // DiffEngine_MaxInstances above that threw out of this form's constructor, so Options
+        // could never be opened to lower it again.
+        this.maxInstancesNumericUpDown.Maximum = new decimal(new int[]
+        {
+            65535,
+            0,
+            0,
+            0
+        });
         this.maxInstancesNumericUpDown.Minimum = new decimal(new int[]
         {
             0,