diff --git a/src/FastExpressionCompiler/FastExpressionCompiler.cs b/src/FastExpressionCompiler/FastExpressionCompiler.cs
index cc503103..6a977c65 100644
--- a/src/FastExpressionCompiler/FastExpressionCompiler.cs
+++ b/src/FastExpressionCompiler/FastExpressionCompiler.cs
@@ -8562,6 +8562,8 @@ internal static bool TryInterpretBool(ref bool resultBool, Expression expr, Expr
                 // not: negate, arithmetic, convert to bool
                 if (nodeType == ExpressionType.Not)
                 {
+                    if (((UnaryExpression)expr).Method != null)
+                        return false;
                     var operandExpr = ((UnaryExpression)expr).Operand;
                     if (operandExpr.NodeType == ExpressionType.Conditional)
                         operandExpr = Tools.TryReduceConditional((ConditionalExpression)operandExpr);
@@ -9583,7 +9585,7 @@ public static T GetFirst<T>(this IEnumerable<T> source)
         public static Expression TryReduceConditionalTest(Expression testExpr)
         {
             // removing Not by turning Equal -> NotEqual, NotEqual -> Equal
-            if (testExpr.NodeType == ExpressionType.Not)
+            if (testExpr.NodeType == ExpressionType.Not && ((UnaryExpression)testExpr).Method == null)
             {
                 // simplify the not `==` -> `!=`, `!=` -> `==`
                 var op = TryReduceConditionalTest(((UnaryExpression)testExpr).Operand);
diff --git a/test/FastExpressionCompiler.TestsRunner.Net472/Program.cs b/test/FastExpressionCompiler.TestsRunner.Net472/Program.cs
index 0e3921b6..9cf16cc2 100644
--- a/test/FastExpressionCompiler.TestsRunner.Net472/Program.cs
+++ b/test/FastExpressionCompiler.TestsRunner.Net472/Program.cs
@@ -45,6 +45,7 @@ static void RunLightExpressionTests(object state)
                 t.Run(new LightExpression.IssueTests.Issue499_InvalidProgramException_for_Sorting_and_comparison_function());
                 t.Run(new LightExpression.UnitTests.ByRefConversionRegressionTests());
                 t.Run(new LightExpression.UnitTests.RuntimeTypeConstantRegressionTests());
+                t.Run(new LightExpression.UnitTests.MethodBasedNotRegressionTests());
                 t.Run(new LightExpression.UnitTests.ExtensionReductionIdentityRegressionTests());
                 t.Run(new LightExpression.UnitTests.ParityTests());
 
@@ -69,6 +70,7 @@ static void RunLightExpressionTests(object state)
             fecTests.Run(new ReductionProgressRegressionTests());
             fecTests.Run(new ByRefConversionRegressionTests());
             fecTests.Run(new RuntimeTypeConstantRegressionTests());
+            fecTests.Run(new MethodBasedNotRegressionTests());
             fecTests.Run(new CustomReducibleNodeRegressionTests());
             fecTests.Run(new ExtensionReductionIdentityRegressionTests());
             fecTests.Run(new ParityTests());
diff --git a/test/FastExpressionCompiler.TestsRunner/Program.cs b/test/FastExpressionCompiler.TestsRunner/Program.cs
index ef6fc328..f10a72a2 100644
--- a/test/FastExpressionCompiler.TestsRunner/Program.cs
+++ b/test/FastExpressionCompiler.TestsRunner/Program.cs
@@ -77,6 +77,7 @@ public static void Main(string[] args)
                 lt.Run(new LightExpression.IssueTests.Issue561_NonBooleanEqualityOperator());
                 lt.Run(new LightExpression.UnitTests.ByRefConversionRegressionTests());
                 lt.Run(new LightExpression.UnitTests.RuntimeTypeConstantRegressionTests());
+                lt.Run(new LightExpression.UnitTests.MethodBasedNotRegressionTests());
                 lt.Run(new LightExpression.UnitTests.ExtensionReductionIdentityRegressionTests());
                 lt.Run(new LightExpression.UnitTests.ParityTests());
                 ltTime = ltStopwatch.ElapsedMilliseconds;
@@ -88,6 +89,7 @@ public static void Main(string[] args)
             st.Run(new ReductionProgressRegressionTests());
             st.Run(new ByRefConversionRegressionTests());
             st.Run(new RuntimeTypeConstantRegressionTests());
+            st.Run(new MethodBasedNotRegressionTests());
             st.Run(new CustomReducibleNodeRegressionTests());
             st.Run(new ExtensionReductionIdentityRegressionTests());
             st.Run(new ParityTests());
diff --git a/test/FastExpressionCompiler.UnitTests/MethodBasedNotRegressionTests.cs b/test/FastExpressionCompiler.UnitTests/MethodBasedNotRegressionTests.cs
new file mode 100644
index 00000000..c4462db4
--- /dev/null
+++ b/test/FastExpressionCompiler.UnitTests/MethodBasedNotRegressionTests.cs
@@ -0,0 +1,87 @@
+using System;
+using SysExpr = System.Linq.Expressions.Expression;
+
+#if LIGHT_EXPRESSION
+namespace FastExpressionCompiler.LightExpression.UnitTests;
+#else
+namespace FastExpressionCompiler.UnitTests;
+#endif
+
+public struct MethodBasedNotRegressionTests : ITestX
+{
+    public void Run(TestRun t)
+    {
+        Explicit_not_method_controls_result(t);
+        Conditional_preserves_custom_not_semantics(t);
+        Ignored_result_preserves_custom_not_effects(t);
+        Nullable_method_receives_null(t);
+    }
+
+    public void Explicit_not_method_controls_result(TestContext t)
+    {
+        var parameter = SysExpr.Parameter(typeof(bool));
+        var method = typeof(MethodBasedNotRegressionTests).GetMethod(nameof(Identity));
+        foreach (var flags in new[] { CompilerFlags.Default, CompilerFlags.DisableInterpreter })
+        foreach (var value in new[] { false, true })
+        {
+            var constant = SysExpr.Lambda<Func<bool>>(SysExpr.Not(SysExpr.Constant(value), method));
+            var fastConstant = constant.FromSysExpression().CompileFast(true, flags);
+            if (!t.IsNotNull(fastConstant)) return;
+            t.AreEqual(value, fastConstant());
+            var expression = SysExpr.Lambda<Func<bool, bool>>(SysExpr.Not(parameter, method), parameter);
+            var fast = expression.FromSysExpression().CompileFast(true, flags);
+            if (!t.IsNotNull(fast)) return;
+            t.AreEqual(value, fast(value));
+        }
+    }
+
+    public void Conditional_preserves_custom_not_semantics(TestContext t)
+    {
+        var parameter = SysExpr.Parameter(typeof(bool));
+        var method = typeof(MethodBasedNotRegressionTests).GetMethod(nameof(Identity));
+        foreach (var flags in new[] { CompilerFlags.Default, CompilerFlags.DisableInterpreter })
+        foreach (var value in new[] { false, true })
+        foreach (var shape in new[] { 0, 1, 2 })
+        {
+            System.Linq.Expressions.Expression operand = shape == 0 ? SysExpr.Constant(value)
+                : shape == 1 ? parameter : SysExpr.Equal(parameter, SysExpr.Constant(true));
+            var body = SysExpr.Condition(SysExpr.Not(operand, method), SysExpr.Constant(42), SysExpr.Constant(7));
+            var expression = SysExpr.Lambda<Func<bool, int>>(body, parameter);
+            var fast = expression.FromSysExpression().CompileFast(true, flags);
+            if (!t.IsNotNull(fast)) return;
+            Calls = 0;
+            t.AreEqual(expression.Compile()(value), fast(value));
+            t.AreEqual(2, Calls);
+        }
+    }
+
+    public void Ignored_result_preserves_custom_not_effects(TestContext t)
+    {
+        var method = typeof(MethodBasedNotRegressionTests).GetMethod(nameof(Identity));
+        var expression = SysExpr.Lambda<Func<int>>(SysExpr.Block(
+            SysExpr.Not(SysExpr.Constant(true), method), SysExpr.Constant(7)));
+        foreach (var flags in new[] { CompilerFlags.Default, CompilerFlags.DisableInterpreter })
+        {
+            var fast = expression.FromSysExpression().CompileFast(true, flags);
+            if (!t.IsNotNull(fast)) return;
+            Calls = 0;
+            t.AreEqual(7, fast());
+            t.AreEqual(1, Calls);
+        }
+    }
+
+    public void Nullable_method_receives_null(TestContext t)
+    {
+        var parameter = SysExpr.Parameter(typeof(bool?));
+        var method = typeof(MethodBasedNotRegressionTests).GetMethod(nameof(NullableIdentity));
+        var expression = SysExpr.Lambda<Func<bool?, bool?>>(SysExpr.Not(parameter, method), parameter);
+        var fast = expression.FromSysExpression().CompileFast(true);
+        if (!t.IsNotNull(fast)) return;
+        foreach (var value in new bool?[] { null, false, true })
+            t.AreEqual(expression.Compile()(value), fast(value));
+    }
+
+    public static int Calls;
+    public static bool Identity(bool value) { ++Calls; return value; }
+    public static bool? NullableIdentity(bool? value) { ++Calls; return value ?? true; }
+}