diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 95e0574562a2d..b2b44c29bdf24 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -623,6 +623,8 @@ Bug Fixes to C++ Support
 - Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)
 - Fix a crash when checking the template template parameters of a dependent lambda appearing in an alias declaration.
   (#GH136432), (#GH137014), (#GH138018)
+- Fixed an assertion when trying to constant-fold various builtins when the argument
+  refered to a reference to an incomplete type. (#GH129397)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -769,7 +771,7 @@ clang-format
 
 libclang
 --------
-- Fixed a bug in ``clang_File_isEqual`` that sometimes led to different 
+- Fixed a bug in ``clang_File_isEqual`` that sometimes led to different
   in-memory files to be considered as equal.
 - Added ``clang_visitCXXMethods``, which allows visiting the methods
   of a class.
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index f2e49b9ea669e..b79d8c197fe7d 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -12772,11 +12772,13 @@ static bool determineEndOffset(EvalInfo &Info, SourceLocation ExprLoc,
   bool DetermineForCompleteObject = refersToCompleteObject(LVal);
 
   auto CheckedHandleSizeof = [&](QualType Ty, CharUnits &Result) {
-    if (Ty.isNull() || Ty->isIncompleteType() || Ty->isFunctionType())
+    if (Ty.isNull())
       return false;
 
-    if (Ty->isReferenceType())
-      Ty = Ty.getNonReferenceType();
+    Ty = Ty.getNonReferenceType();
+
+    if (Ty->isIncompleteType() || Ty->isFunctionType())
+      return false;
 
     return HandleSizeof(Info, ExprLoc, Ty, Result);
   };
diff --git a/clang/test/SemaCXX/builtin-object-size-cxx14.cpp b/clang/test/SemaCXX/builtin-object-size-cxx14.cpp
index b7c6f6be01f54..fdd3cb7af088f 100644
--- a/clang/test/SemaCXX/builtin-object-size-cxx14.cpp
+++ b/clang/test/SemaCXX/builtin-object-size-cxx14.cpp
@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx14 -std=c++14 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++2a %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2b %s
+
 
 typedef __SIZE_TYPE__ size_t;
 
@@ -119,3 +121,13 @@ constexpr int bos_new() { // cxx14-error {{constant expression}}
   void *p = new int; // cxx14-note {{until C++20}}
   return __builtin_object_size(p, 0);
 }
+
+
+namespace GH129397 {
+
+struct incomplete;
+void test(incomplete &ref) {
+  __builtin_object_size(&ref, 1);
+}
+
+}