diff --git a/Sources/Tests/UnitTests/Core/Multithreading/SettingsAndThreads.cs b/Sources/Tests/UnitTests/Core/Multithreading/SettingsAndThreads.cs
index 99a6eacfd..934c7945c 100644
--- a/Sources/Tests/UnitTests/Core/Multithreading/SettingsAndThreads.cs
+++ b/Sources/Tests/UnitTests/Core/Multithreading/SettingsAndThreads.cs
@@ -54,7 +54,8 @@ static void Solve(int num) =>
                 {
                     var roots = "x2 + e^x + sin(x)".SolveEquation("x");
                     var set = (Set)roots.Simplify();
-                    Assert.Equal(num % 2 == 0, !set.IsSetEmpty);
+                    // With Newton the roots are numbers; without, the equation is left as the set it defines.
+                    Assert.Equal(num % 2 == 0, set is Set.FiniteSet { IsSetEmpty: false });
                 });
             new ThreadingChecker(Solve).Run();
         }
@@ -123,7 +124,8 @@ static void Solve(int num)
                 using var _ = MathS.Settings.AllowNewton.Set(num % 2 == 0);
                 var roots = "x2 + e^x + sin(x)".SolveEquation("x");
                 var set = (Set)roots.Simplify();
-                Assert.Equal(num % 2 == 0, !set.IsSetEmpty);
+                // With Newton the roots are numbers; without, the equation is left as the set it defines.
+                Assert.Equal(num % 2 == 0, set is Set.FiniteSet { IsSetEmpty: false });
             }
             new ThreadingChecker(Solve).Run();
         }
diff --git a/Sources/Tests/UnitTests/TestUtils.cs b/Sources/Tests/UnitTests/TestUtils.cs
index e5aca28a7..7eea6227a 100644
--- a/Sources/Tests/UnitTests/TestUtils.cs
+++ b/Sources/Tests/UnitTests/TestUtils.cs
@@ -135,14 +135,19 @@ public void Run(int iterCount = 1, int threadCount = 4)
         {
             var tasks = new Task[threadCount];
             for (var i = 0; i < threadCount; i++)
+            {
+                // Each task's own index: the lambda read the loop's variable, which had usually reached
+                // threadCount by the time a task ran, so every thread was asked the same thing.
+                var index = i;
                 tasks[i] = Task.Run(
                     () =>
                     {
                         var iterCountLocal = iterCount;
                         for (var j = 0; j < iterCountLocal; j++)
-                            action(i);
+                            action(index);
                     }
                 );
+            }
             Task.WaitAll(tasks);
         }
     }