diff --git a/llvm/lib/Demangle/MicrosoftDemangle.cpp b/llvm/lib/Demangle/MicrosoftDemangle.cpp
--- a/llvm/lib/Demangle/MicrosoftDemangle.cpp
+++ b/llvm/lib/Demangle/MicrosoftDemangle.cpp
@@ -2404,7 +2404,8 @@
   std::string_view Name{MangledName};
   SymbolNode *AST = D.parse(Name);
   if (!D.Error && NMangled)
-    *NMangled = Name.empty() ? 0 : &*Name.begin() - &*MangledName.begin();
+    *NMangled = Name.empty() ? MangledName.size()
+                             : &*Name.begin() - &*MangledName.begin();
 
   if (Flags & MSDF_DumpBackrefs)
     D.dumpBackReferences();
diff --git a/llvm/test/Demangle/warn-trailing.test b/llvm/test/Demangle/warn-trailing.test
--- a/llvm/test/Demangle/warn-trailing.test
+++ b/llvm/test/Demangle/warn-trailing.test
@@ -4,3 +4,8 @@
 ; CHECK: ?x@@3HAasdf
 ; CHECK-NEXT: int x
 ; CHECK-NEXT: warning: trailing characters: asdf
+
+.?AVNet@@
+; CHECK: .?AVNet@@
+; CHECK-NEXT: class Net `RTTI Type Descriptor Name'
+; CHECK-NOT: warning: trailing characters