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
@@ -4866,6 +4866,11 @@
       if (Result.getAsSingle<TypeDecl>() ||
           Result.getResultKind() ==
               LookupResult::NotFoundInCurrentInstantiation) {
+        if (SS.isEmpty())
+          // bail out if we don't have a NNS, this could be happened during
+          // typo correction in error recovery.
+          // A dependent name type should have a non-null NNS.
+          return true;
         // Suggest that the user add 'typename' before the NNS.
         SourceLocation Loc = AL.getSourceRange().getBegin();
         Diag(Loc, getLangOpts().MSVCCompat
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
@@ -286,3 +286,16 @@
   template<int> int b;
   template<int> auto f() -> b<0>; // expected-error +{{}}
 }
+
+namespace NoCrashOnNullNNSTypoCorrection {
+
+int AddObservation(); // expected-note {{declared here}}
+
+template <typename T, typename... Args>
+class UsingImpl {};
+class AddObservation {
+  using Using =
+    UsingImpl<AddObservationFn, const int>; // expected-error {{use of undeclared identifier 'AddObservationFn'; did you mean}}
+};
+
+}