Index: include/llvm/Bitcode/BitstreamWriter.h
===================================================================
--- include/llvm/Bitcode/BitstreamWriter.h
+++ include/llvm/Bitcode/BitstreamWriter.h
@@ -300,6 +300,7 @@
     unsigned BlobLen = (unsigned) Blob.size();
     unsigned AbbrevNo = Abbrev-bitc::FIRST_APPLICATION_ABBREV;
     assert(AbbrevNo < CurAbbrevs.size() && "Invalid abbrev #!");
+    assert(Abbrev < (1U << CurCodeSize) && "Block code length is too small");
     const BitCodeAbbrev *Abbv = CurAbbrevs[AbbrevNo].get();
 
     EmitCode(Abbrev);
Index: unittests/Bitcode/BitstreamWriterTest.cpp
===================================================================
--- unittests/Bitcode/BitstreamWriterTest.cpp
+++ unittests/Bitcode/BitstreamWriterTest.cpp
@@ -56,4 +56,33 @@
   EXPECT_EQ(StringRef("str0"), Buffer);
 }
 
+#ifndef NDEBUG
+TEST(BitstreamWriterTest, trapOnSmallAbbrev) {
+  SmallString<64> Buffer;
+  BitstreamWriter W(Buffer);
+  W.EnterSubblock(bitc::FIRST_APPLICATION_BLOCKID, /*CodeLen*/2);
+  auto Abbrev = std::make_shared<BitCodeAbbrev>();
+  Abbrev->Add(BitCodeAbbrevOp(42U));
+  unsigned AbbrevCode = W.EmitAbbrev(std::move(Abbrev));
+  EXPECT_DEATH({ W.EmitRecordWithAbbrev(AbbrevCode, makeArrayRef(42U)); },
+               "Block code length is too small");
+  W.ExitBlock();
+}
+
+TEST(BitstreamWriterTest, trapOnSmallAbbrevUsingBlockInfo) {
+  SmallString<64> Buffer;
+  BitstreamWriter W(Buffer);
+  W.EnterBlockInfoBlock();
+  auto Abbrev = std::make_shared<BitCodeAbbrev>();
+  Abbrev->Add(BitCodeAbbrevOp(42U));
+  unsigned AbbrevCode = W.EmitBlockInfoAbbrev(bitc::FIRST_APPLICATION_BLOCKID,
+                                              std::move(Abbrev));
+  W.ExitBlock();
+  W.EnterSubblock(bitc::FIRST_APPLICATION_BLOCKID, /*CodeLen*/2);
+  EXPECT_DEATH({ W.EmitRecordWithAbbrev(AbbrevCode, makeArrayRef(42U)); },
+               "Block code length is too small");
+  W.ExitBlock();
+}
+#endif
+
 } // end namespace