diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -5928,7 +5928,9 @@
 
 bool UnnamedLocalNoLinkageFinder::VisitDependentTemplateSpecializationType(
                                  const DependentTemplateSpecializationType* T) {
-  return VisitNestedNameSpecifier(T->getQualifier());
+  if (auto *Q = T->getQualifier())
+    return VisitNestedNameSpecifier(Q);
+  return false;
 }
 
 bool UnnamedLocalNoLinkageFinder::VisitPackExpansionType(
@@ -5982,6 +5984,7 @@
 
 bool UnnamedLocalNoLinkageFinder::VisitNestedNameSpecifier(
                                                     NestedNameSpecifier *NNS) {
+  assert(NNS);
   if (NNS->getPrefix() && VisitNestedNameSpecifier(NNS->getPrefix()))
     return true;
 
diff --git a/clang/test/Parser/cxx-template-decl.cpp b/clang/test/Parser/cxx-template-decl.cpp
--- a/clang/test/Parser/cxx-template-decl.cpp
+++ b/clang/test/Parser/cxx-template-decl.cpp
@@ -273,3 +273,9 @@
 namespace PR45063 {
   template<class=class a::template b<>> struct X {}; // expected-error {{undeclared identifier 'a'}}
 }
+
+namespace NoCrashOnEmptyNestedNameSpecifier {
+  template <typename FnT,
+            typename T = typename ABC<FnT>::template arg_t<0>> // expected-error {{no template named 'ABC'}}
+  void foo(FnT) {}
+}