diff --git a/include/llvm/Object/Archive.h b/include/llvm/Object/Archive.h
index 597f0d48c11..e5d59d832e3 100644
--- a/include/llvm/Object/Archive.h
+++ b/include/llvm/Object/Archive.h
@@ -74,6 +74,7 @@ public:
       return Data.begin() < other.Data.begin();
     }
 
+    const Archive *getParent() const { return Parent; }
     Child getNext() const;
 
     ErrorOr<StringRef> getName() const;
diff --git a/lib/Object/ArchiveWriter.cpp b/lib/Object/ArchiveWriter.cpp
index bcd5382d91e..b555b8cfdcf 100644
--- a/lib/Object/ArchiveWriter.cpp
+++ b/lib/Object/ArchiveWriter.cpp
@@ -318,6 +318,9 @@ llvm::writeArchive(StringRef ArcName,
       MemberRef = Buffers.back()->getMemBufferRef();
     } else {
       object::Archive::child_iterator OldMember = Member.getOld();
+      assert(!Thin ||
+             OldMember->getParent()->isThin() &&
+                 "Thin archives cannot refers to member of other archives");
       ErrorOr<MemoryBufferRef> MemberBufferOrErr =
           OldMember->getMemoryBufferRef();
       if (auto EC = MemberBufferOrErr.getError())
diff --git a/test/Object/archive-update.test b/test/Object/archive-update.test
index 147db90ba95..ef1a71f60f4 100644
--- a/test/Object/archive-update.test
+++ b/test/Object/archive-update.test
@@ -34,3 +34,17 @@ RUN: llvm-ar p %t.a | FileCheck --check-prefix=NEWER %s
 
 NEWER: newer
 OLDER: older
+
+RUN: rm -f %t.a
+RUN: echo foo > foo
+RUN: echo bar > bar
+RUN: llvm-ar --format=gnu rcT %t.a foo
+RUN: llvm-ar --format=gnu rcT %t.a bar
+RUN: llvm-ar t %t.a | FileCheck --check-prefix=BOTH-FILES %s
+BOTH-FILES: foo
+BOTH-FILES: bar
+
+RUN: rm -f %t.a
+RUN: llvm-ar --format=gnu rc %t.a foo
+RUN: not llvm-ar --format=gnu rcT %t.a bar 2>&1 | FileCheck --check-prefix=ERROR %s
+ERROR: Cannot convert a regular archive to a thin one.
diff --git a/tools/llvm-ar/llvm-ar.cpp b/tools/llvm-ar/llvm-ar.cpp
index 0e3c4fdc2ce..92f272dfad1 100644
--- a/tools/llvm-ar/llvm-ar.cpp
+++ b/tools/llvm-ar/llvm-ar.cpp
@@ -436,9 +436,20 @@ static void performReadOperation(ArchiveOperation Operation,
   std::exit(1);
 }
 
-template <typename T>
-void addMember(std::vector<NewArchiveIterator> &Members, T I, StringRef Name,
+void addMember(std::vector<NewArchiveIterator> &Members, StringRef FileName,
+               StringRef Name, int Pos = -1) {
+  NewArchiveIterator NI(FileName, Name);
+  if (Pos == -1)
+    Members.push_back(NI);
+  else
+    Members[Pos] = NI;
+}
+
+void addMember(std::vector<NewArchiveIterator> &Members,
+               object::Archive::child_iterator I, StringRef Name,
                int Pos = -1) {
+  if (Thin && !I->getParent()->isThin())
+    fail("Cannot convert a regular archive to a thin one");
   NewArchiveIterator NI(I, Name);
   if (Pos == -1)
     Members.push_back(NI);