diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 37a664b14fab1f..055ac980e530d2 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -802,6 +802,7 @@ Bug Fixes to C++ Support
 - Fixed a regression introduced in Clang 18 causing a static function overloading a non-static function
   with the same parameters not to be diagnosed. (Fixes #GH93456).
 - Clang now diagnoses unexpanded parameter packs in attributes. (Fixes #GH93269).
+- Fix an assertion failure when checking invalid ``this`` usage in the wrong context. (Fixes #GH91536).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index d3e9dcb4f4399a..6595abbcdda5b1 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1444,10 +1444,10 @@ bool Sema::CheckCXXThisType(SourceLocation Loc, QualType Type) {
   //   category are defined within such member functions as they are within
   //   an implicit object member function).
   DeclContext *DC = getFunctionLevelDeclContext();
-  if (const auto *Method = dyn_cast<CXXMethodDecl>(DC);
-      Method && Method->isExplicitObjectMemberFunction()) {
+  const auto *Method = dyn_cast<CXXMethodDecl>(DC);
+  if (Method && Method->isExplicitObjectMemberFunction()) {
     Diag(Loc, diag::err_invalid_this_use) << 1;
-  } else if (isLambdaCallWithExplicitObjectParameter(CurContext)) {
+  } else if (Method && isLambdaCallWithExplicitObjectParameter(CurContext)) {
     Diag(Loc, diag::err_invalid_this_use) << 1;
   } else {
     Diag(Loc, diag::err_invalid_this_use) << 0;
diff --git a/clang/test/SemaCXX/invalid-this-in-lambda.cpp b/clang/test/SemaCXX/invalid-this-in-lambda.cpp
new file mode 100644
index 00000000000000..ae65bda025e232
--- /dev/null
+++ b/clang/test/SemaCXX/invalid-this-in-lambda.cpp
@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+
+decltype([]()->decltype(this) { }) a; // expected-error {{invalid use of 'this' outside of a non-static member function}}
+