diff --git a/llvm/include/llvm/MC/MCParser/AsmLexer.h b/llvm/include/llvm/MC/MCParser/AsmLexer.h
index 11d32fbb64702..e4cdb2ccd2e6e 100644
--- a/llvm/include/llvm/MC/MCParser/AsmLexer.h
+++ b/llvm/include/llvm/MC/MCParser/AsmLexer.h
@@ -45,6 +45,7 @@ class AsmLexer {
   SmallVector<AsmToken, 1> CurTok;
 
   const char *CurPtr = nullptr;
+  /// NULL-terminated buffer. NULL terminator must reside at `CurBuf.end()`.
   StringRef CurBuf;
 
   /// The location and description of the current error
@@ -191,6 +192,12 @@ class AsmLexer {
   /// literals.
   void setLexHLASMStrings(bool V) { LexHLASMStrings = V; }
 
+  /// Set buffer to be lexed.
+  /// `Buf` must be NULL-terminated. NULL terminator must reside at `Buf.end()`.
+  /// `ptr` if provided must be in range [`Buf.begin()`, `buf.end()`] or NULL.
+  /// Specifies where lexing of buffer should begin.
+  /// `EndStatementAtEOF` specifies whether `AsmToken::EndOfStatement` should be
+  /// returned upon reaching end of buffer.
   LLVM_ABI void setBuffer(StringRef Buf, const char *ptr = nullptr,
                           bool EndStatementAtEOF = true);
 
diff --git a/llvm/lib/MC/MCParser/AsmLexer.cpp b/llvm/lib/MC/MCParser/AsmLexer.cpp
index 968ccf776440b..6a89c1d09af09 100644
--- a/llvm/lib/MC/MCParser/AsmLexer.cpp
+++ b/llvm/lib/MC/MCParser/AsmLexer.cpp
@@ -120,6 +120,11 @@ AsmLexer::AsmLexer(const MCAsmInfo &MAI) : MAI(MAI) {
 
 void AsmLexer::setBuffer(StringRef Buf, const char *ptr,
                          bool EndStatementAtEOF) {
+  // Buffer must be NULL-terminated. NULL terminator must reside at `Buf.end()`.
+  // It must be safe to dereference `Buf.end()`.
+  assert(*Buf.end() == '\0' &&
+         "Buffer provided to AsmLexer lacks null terminator.");
+
   CurBuf = Buf;
 
   if (ptr)