diff --git a/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Diagnostics.xml b/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Diagnostics.xml
new file mode 100644
index 0000000..328f7a0
--- /dev/null
+++ b/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Diagnostics.xml
@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<diagnostics id="WTG3013" message="Don't use Concat when appending a single element to an enumerable." severity="Info">
+	<diagnostic>
+		<location>Test0.cs: (8, 32-57)</location>
+	</diagnostic>
+</diagnostics>
diff --git a/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Result.cs b/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Result.cs
new file mode 100644
index 0000000..cdefe23
--- /dev/null
+++ b/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Result.cs
@@ -0,0 +1,13 @@
+using System.Collections.Generic;
+using System.Linq;
+
+public class Bob
+{
+	public int[] Method(Bob other)
+	{
+		var items = other.GetItems()?.Where(x => x > 0).Append(42).ToArray();
+		return items;
+	}
+
+	public IEnumerable<int> GetItems() => new[] { 1, 2, 3 };
+}
diff --git a/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Source.cs b/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Source.cs
new file mode 100644
index 0000000..52f6dde
--- /dev/null
+++ b/src/WTG.Analyzers.Test/TestData/LinqEnumerableAnalyzer/ConditionalAccess/Source.cs
@@ -0,0 +1,13 @@
+using System.Collections.Generic;
+using System.Linq;
+
+public class Bob
+{
+	public int[] Method(Bob other)
+	{
+		var items = other.GetItems()?.Where(x => x > 0).Concat(new[] { 42 }).ToArray();
+		return items;
+	}
+
+	public IEnumerable<int> GetItems() => new[] { 1, 2, 3 };
+}
diff --git a/src/WTG.Analyzers/Analyzers/LinqEnumerable/LinqEnumerableCodeFixProvider.cs b/src/WTG.Analyzers/Analyzers/LinqEnumerable/LinqEnumerableCodeFixProvider.cs
index 18ea77f..ab12fe0 100644
--- a/src/WTG.Analyzers/Analyzers/LinqEnumerable/LinqEnumerableCodeFixProvider.cs
+++ b/src/WTG.Analyzers/Analyzers/LinqEnumerable/LinqEnumerableCodeFixProvider.cs
@@ -121,9 +121,7 @@ public static SyntaxNode FixConcatWithAppendMethod(MemberAccessExpressionSyntax
 			return InvocationExpression(
 				MemberAccessExpression(
 					SyntaxKind.SimpleMemberAccessExpression,
-					ParenthesizedExpression(m.Expression.WithoutTrivia())
-						.WithTriviaFrom(m.Expression)
-						.WithAdditionalAnnotations(Simplifier.Annotation),
+					WrapExpressionIfNeeded(m.Expression),
 					m.OperatorToken,
 					IdentifierName(nameof(Enumerable.Append))
 						.WithTriviaFrom(m.Name)))
@@ -146,9 +144,7 @@ public static SyntaxNode FixConcatWithAppendMethod(MemberAccessExpressionSyntax
 			{
 				case 1:
 					listOfArgumentsAndSeparators.Add(Argument(LinqEnumerableUtils.GetFirstValue(m.Expression.TryGetExpressionFromParenthesizedExpression())!));
-					member = ParenthesizedExpression(invocation.ArgumentList.Arguments[0].Expression.WithoutTrivia())
-						.WithTriviaFrom(m.Expression)
-						.WithAdditionalAnnotations(Simplifier.Annotation);
+					member = WrapExpressionIfNeeded(invocation.ArgumentList.Arguments[0].Expression.WithTriviaFrom(m.Expression));
 					break;
 				case 2:
 					listOfArgumentsAndSeparators.Add(invocation.ArgumentList.Arguments[1]);
@@ -213,5 +209,22 @@ public static SyntaxNode FixConcatWithNewCollection(MemberAccessExpressionSyntax
 					.WithTriviaFrom(invocation)
 					.WithAdditionalAnnotations(Simplifier.Annotation);
 		}
+
+		static ExpressionSyntax WrapExpressionIfNeeded(ExpressionSyntax expression)
+		{
+			// Invocation expressions never need parenthesization as they already have
+			// clear binding. Wrapping them can cause issues when the expression is part
+			// of a conditional access chain (?.), as it disconnects MemberBindingExpressions
+			// from their ConditionalAccessExpression, causing Roslyn's speculative semantic
+			// model to crash with a NullReferenceException.
+			if (expression.IsKind(SyntaxKind.InvocationExpression))
+			{
+				return expression;
+			}
+
+			return ParenthesizedExpression(expression.WithoutTrivia())
+				.WithTriviaFrom(expression)
+				.WithAdditionalAnnotations(Simplifier.Annotation);
+		}
 	}
 }