diff --git a/src/PolyType.SourceGenerator/SourceFormatter/SourceFormatter.Function.cs b/src/PolyType.SourceGenerator/SourceFormatter/SourceFormatter.Function.cs
index e26efc73..9ffec637 100644
--- a/src/PolyType.SourceGenerator/SourceFormatter/SourceFormatter.Function.cs
+++ b/src/PolyType.SourceGenerator/SourceFormatter/SourceFormatter.Function.cs
@@ -176,7 +176,7 @@ static string FormatFunctionParameterExpr(ParameterShapeModel parameter, bool is
             }
 
             string delegateSignature = string.Join(", ", functionShapeModel.Parameters
-                .Select(parameter => $"{FormatRefPrefix(parameter)}{parameter.ParameterType.FullyQualifiedName}{GetNullableSuffix(parameter)} {parameter.Name}"));
+                .Select(parameter => $"{FormatRefPrefix(parameter)}{FormatDelegateParameterType(parameter)} {parameter.Name}"));
 
             string argumentStateCtorExpr = functionShapeModel.Parameters switch
             {
@@ -199,7 +199,19 @@ static string FormatFunctionParameterExpr(ParameterShapeModel parameter, bool is
 
             return $$"""static {{innerFuncVar}} => ({{delegateSignature}}) => { {{functionArgumentStateFQN}} {{stateVar}} = {{argumentStateCtorExpr}}; {{tailExpr}}; }""";
 
-            static string GetNullableSuffix(ParameterShapeModel parameter) => parameter.NullableAnnotation is NullableAnnotation.Annotated ? "?" : "";
+            static string FormatDelegateParameterType(ParameterShapeModel parameter)
+            {
+                string parameterType = parameter.ParameterType.FullyQualifiedName;
+                if (parameter.NullableAnnotation is not NullableAnnotation.Annotated)
+                {
+                    return parameterType;
+                }
+
+                return parameterType.Length > 0 && parameterType[^1] == '?'
+                    ? parameterType
+                    : parameterType + "?";
+            }
+
             static string GetSuppressionSuffix(ParameterShapeModel parameter) => parameter.ParameterTypeContainsNullabilityAnnotations ? "!" : "";
         }
 
diff --git a/tests/PolyType.SourceGenerator.UnitTests/CompilationTests.FunctionShapes.cs b/tests/PolyType.SourceGenerator.UnitTests/CompilationTests.FunctionShapes.cs
index 7a26a970..86b7dc93 100644
--- a/tests/PolyType.SourceGenerator.UnitTests/CompilationTests.FunctionShapes.cs
+++ b/tests/PolyType.SourceGenerator.UnitTests/CompilationTests.FunctionShapes.cs
@@ -43,6 +43,23 @@ public partial class Witness { }
         Assert.Empty(result.Diagnostics);
     }
 
+    [Fact]
+    public static void DelegateShapes_NullableBoolParameter_NoErrors()
+    {
+        // Regression test for nullable bool delegate parameters being emitted as bool??.
+        Compilation compilation = CompilationHelpers.CreateCompilation("""
+            using PolyType;
+
+            public delegate void BrokenCommand(bool? skipDuplicate = null);
+
+            [GenerateShapeFor(typeof(BrokenCommand))]
+            public partial class Witness { }
+            """);
+
+        PolyTypeSourceGeneratorResult result = CompilationHelpers.RunPolyTypeSourceGenerator(compilation);
+        Assert.Empty(result.Diagnostics);
+    }
+
     [Fact]
     public static void EventShapes_PublicInstanceAndStatic_NoErrors()
     {