Index: libcxxabi/src/demangle/Utility.h
===================================================================
--- libcxxabi/src/demangle/Utility.h
+++ libcxxabi/src/demangle/Utility.h
@@ -75,7 +75,10 @@
   OutputBuffer(const OutputBuffer &) = delete;
   OutputBuffer &operator=(const OutputBuffer &) = delete;
 
-  operator StringView() const { return StringView(Buffer, CurrentPosition); }
+  operator StringView() const {
+    assert(Buffer && "Buffer cannot by nullptr");
+    return StringView(Buffer, CurrentPosition);
+  }
 
   void reset(char *Buffer_, size_t BufferCapacity_) {
     CurrentPosition = 0;
@@ -174,6 +177,8 @@
     return CurrentPosition ? Buffer[CurrentPosition - 1] : '\0';
   }
 
+  // Some uses require a buffer even when they write nothing.
+  void allocateBuffer() { grow(1); }
   bool empty() const { return CurrentPosition == 0; }
 
   char *getBuffer() { return Buffer; }
Index: llvm/include/llvm/Demangle/Utility.h
===================================================================
--- llvm/include/llvm/Demangle/Utility.h
+++ llvm/include/llvm/Demangle/Utility.h
@@ -75,7 +75,10 @@
   OutputBuffer(const OutputBuffer &) = delete;
   OutputBuffer &operator=(const OutputBuffer &) = delete;
 
-  operator StringView() const { return StringView(Buffer, CurrentPosition); }
+  operator StringView() const {
+    assert(Buffer && "Buffer cannot by nullptr");
+    return StringView(Buffer, CurrentPosition);
+  }
 
   void reset(char *Buffer_, size_t BufferCapacity_) {
     CurrentPosition = 0;
@@ -174,6 +177,8 @@
     return CurrentPosition ? Buffer[CurrentPosition - 1] : '\0';
   }
 
+  // Some uses require a buffer even when they write nothing.
+  void allocateBuffer() { grow(1); }
   bool empty() const { return CurrentPosition == 0; }
 
   char *getBuffer() { return Buffer; }
Index: llvm/lib/Demangle/MicrosoftDemangle.cpp
===================================================================
--- llvm/lib/Demangle/MicrosoftDemangle.cpp
+++ llvm/lib/Demangle/MicrosoftDemangle.cpp
@@ -1301,6 +1301,10 @@
     goto StringLiteralError;
   }
 
+  // We can demangle to an empty string, and need to avoid a null buffer in that
+  // case.
+  OB.allocateBuffer();
+
   // Encoded Length
   std::tie(StringByteSize, IsNegative) = demangleNumber(MangledName);
   if (Error || IsNegative || StringByteSize < (IsWcharT ? 2 : 1))
Index: llvm/unittests/Demangle/OutputBufferTest.cpp
===================================================================
--- llvm/unittests/Demangle/OutputBufferTest.cpp
+++ llvm/unittests/Demangle/OutputBufferTest.cpp
@@ -44,6 +44,8 @@
 TEST(OutputBufferTest, Insert) {
   OutputBuffer OB;
 
+  OB.allocateBuffer();
+
   OB.insert(0, "", 0);
   EXPECT_EQ("", toString(OB));