diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -135,7 +135,11 @@
   auto Buf = WritableMemoryBuffer::getNewUninitMemBuffer(InputData.size(), BufferName);
   if (!Buf)
     return make_error_code(errc::not_enough_memory);
-  memcpy(Buf->getBufferStart(), InputData.data(), InputData.size());
+  // Calling memcpy with null src/dst is UB, and an empty StringRef is
+  // represented with {nullptr, 0}. Make sure we don't copy anything in that
+  // case.
+  if (!InputData.empty())
+    memcpy(Buf->getBufferStart(), InputData.data(), InputData.size());
   return std::move(Buf);
 }
 
diff --git a/llvm/unittests/Support/MemoryBufferTest.cpp b/llvm/unittests/Support/MemoryBufferTest.cpp
--- a/llvm/unittests/Support/MemoryBufferTest.cpp
+++ b/llvm/unittests/Support/MemoryBufferTest.cpp
@@ -161,6 +161,10 @@
 
   // verify the two copies do not point to the same place
   EXPECT_NE(MBC1->getBufferStart(), MBC2->getBufferStart());
+
+  // check that copies from defaulted stringrefs don't trigger UB.
+  OwningBuffer MBC3(MemoryBuffer::getMemBufferCopy(StringRef{}));
+  EXPECT_NE(nullptr, MBC3.get());
 }
 
 #if LLVM_ENABLE_THREADS