diff --git a/EasyAssertions/SourceExpressions/SourceExpressionProvider.cs b/EasyAssertions/SourceExpressions/SourceExpressionProvider.cs
index c6b7a85..00b5a93 100644
--- a/EasyAssertions/SourceExpressions/SourceExpressionProvider.cs
+++ b/EasyAssertions/SourceExpressions/SourceExpressionProvider.cs
@@ -7,6 +7,7 @@ class SourceExpressionProvider : ITestExpressionProvider
     List<AssertionFrame> stack = new();
     int currentStackIndex;
     int lastStackIndex;
+    readonly object lockObject = new {};
 
     static readonly AsyncLocal<SourceExpressionProvider> LocalInstance = new();
     public static SourceExpressionProvider Current => LocalInstance.Value ??= new SourceExpressionProvider();
@@ -21,16 +22,19 @@ public void InvokeAssertion(Expression<Action> callAssertionMethod, string actua
 
     void TrackAssertion(Action<IAssertionContext> assert, AssertionCall assertion, string actualSuffix, string expectedSuffix)
     {
-        try
+        lock (lockObject)
         {
-            EnterAssertion(assertion, actualSuffix, expectedSuffix);
-            assert(new AssertionContext());
-            ExitAssertion(true);
-        }
-        catch
-        {
-            ExitAssertion(false);
-            throw;
+            try
+            {
+                EnterAssertion(assertion, actualSuffix, expectedSuffix);
+                assert(new AssertionContext());
+                ExitAssertion(true);
+            }
+            catch
+            {
+                ExitAssertion(false);
+                throw;
+            }
         }
     }
 
@@ -63,8 +67,17 @@ void ExitAssertion(bool success)
             lastStackIndex--;
     }
 
-    public string GetActualExpression() => NormalizeIndentation(LastAssertionFrame?.GetActualExpression() ?? string.Empty);
-    public string GetExpectedExpression() => NormalizeIndentation(LastAssertionFrame?.GetExpectedExpression() ?? string.Empty);
+    public string GetActualExpression()
+    {
+        lock (lockObject)
+            return NormalizeIndentation(LastAssertionFrame?.GetActualExpression() ?? string.Empty);
+    }
+
+    public string GetExpectedExpression()
+    {
+        lock (lockObject)
+            return NormalizeIndentation(LastAssertionFrame?.GetExpectedExpression() ?? string.Empty);
+    }
 
     static string NormalizeIndentation(string input)
     {
@@ -86,4 +99,12 @@ static string NormalizeIndentation(string input)
 
     AssertionFrame? CurrentAssertionFrame => stack.ElementAtOrDefault(currentStackIndex);
     AssertionFrame? LastAssertionFrame => stack.ElementAtOrDefault(lastStackIndex);
+    internal int CurrentStackIndex
+    {
+        get
+        {
+            lock(lockObject)
+                return currentStackIndex;
+        }
+    }
 }
\ No newline at end of file
diff --git a/UnitTests/SourceExpressionProviderTests.cs b/UnitTests/SourceExpressionProviderTests.cs
index 8720717..4a373e4 100644
--- a/UnitTests/SourceExpressionProviderTests.cs
+++ b/UnitTests/SourceExpressionProviderTests.cs
@@ -269,6 +269,26 @@ public void ExpectedExpression_PassedThroughIndexedUserAssertion()
         Assert.AreEqual($"{nameof(expectedExpression)}[{expectedIndex}]", sut.GetExpectedExpression());
     }
 
+    [Test]
+    public void MultiThreads_ShouldNotMessUpStack()
+    {
+        RunTasksParallel(200, TimeSpan.FromSeconds(30), InnerAssertionTask);
+        void InnerAssertionTask()
+        {
+            0.ShouldBe(0);
+            Assert.AreEqual(0, sut.CurrentStackIndex);
+        }
+        void RunTasksParallel(int count, TimeSpan maxRunTime, Action action)
+        {
+            Task[] tasks = Enumerable.Range(0, count)
+                .Select(i => Task.Factory.StartNew(action))
+                .ToArray();
+
+            Task.WaitAll(tasks, maxRunTime).ShouldBe(true);
+            Assert.AreEqual(0, sut.CurrentStackIndex);
+        }
+    }
+
     class TestClass
     {
         public readonly int Value;