diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 7b84210fddab3..262bf4e3d4f5b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -1118,6 +1118,7 @@ Miscellaneous Clang Crashes Fixed
 
 - Fixed a crash when an unscoped enumeration declared by an opaque-enum-declaration within a class template
   with a dependent underlying type is subject to integral promotion. (#GH117960)
+- Fix code completion crash involving PCH serialzied templates. (#GH139019)
 
 OpenACC Specific Changes
 ------------------------
diff --git a/clang/lib/Serialization/TemplateArgumentHasher.cpp b/clang/lib/Serialization/TemplateArgumentHasher.cpp
index 598f098f526d0..5fd6941256fe2 100644
--- a/clang/lib/Serialization/TemplateArgumentHasher.cpp
+++ b/clang/lib/Serialization/TemplateArgumentHasher.cpp
@@ -65,7 +65,9 @@ void TemplateArgumentHasher::AddTemplateArgument(TemplateArgument TA) {
 
   switch (Kind) {
   case TemplateArgument::Null:
-    llvm_unreachable("Expected valid TemplateArgument");
+    // These can occur in incomplete substitutions performed with code
+    // completion (see PartialOverloading).
+    break;
   case TemplateArgument::Type:
     AddQualType(TA.getAsType());
     break;
diff --git a/clang/test/CodeCompletion/GH139019.cpp b/clang/test/CodeCompletion/GH139019.cpp
new file mode 100644
index 0000000000000..fed35b38362a1
--- /dev/null
+++ b/clang/test/CodeCompletion/GH139019.cpp
@@ -0,0 +1,26 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/test.hpp -emit-pch -o %t/1.pch
+// RUN: %clang_cc1 -std=c++20 %t/test.cpp -include-pch %t/1.pch -code-completion-at=%t/test.cpp:7:17
+
+//--- test.hpp
+#pragma once
+class provider_t
+{
+  public:
+    template<class T>
+    void emit(T *data)
+    {}
+};
+
+//--- test.cpp
+#include "test.hpp"
+
+void test()
+{
+    provider_t *focus;
+    void *data;
+    focus->emit(&data);
+}