diff --git a/clang/lib/Sema/SemaCXXScopeSpec.cpp b/clang/lib/Sema/SemaCXXScopeSpec.cpp
index c52fc5bf815af..17ae7ca5627a9 100644
--- a/clang/lib/Sema/SemaCXXScopeSpec.cpp
+++ b/clang/lib/Sema/SemaCXXScopeSpec.cpp
@@ -780,6 +780,11 @@ bool Sema::BuildCXXNestedNameSpecifier(Scope *S, NestedNameSpecInfo &IdInfo,
 
   if (!Found.empty()) {
     const auto *ND = Found.getAsSingle<NamedDecl>();
+    if (!ND) {
+      Diag(IdInfo.IdentifierLoc, diag::err_expected_class_or_namespace)
+          << IdInfo.Identifier << getLangOpts().CPlusPlus;
+      return true;
+    }
     if (::ExtendNestedNameSpecifier(*this, SS, ND, IdInfo.IdentifierLoc,
                                     IdInfo.CCLoc)) {
       const Type *T = SS.getScopeRep().getAsType();
diff --git a/clang/test/Sema/PR166843.cpp b/clang/test/Sema/PR166843.cpp
new file mode 100644
index 0000000000000..5a6223bccc27e
--- /dev/null
+++ b/clang/test/Sema/PR166843.cpp
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -fsyntax-only %s -verify
+namespace a {
+template <class b>
+void c() {
+  ((::c::x)); // expected-error {{'c' is not a class, namespace, or enumeration}}
+}
+}