Index: RawStringLiteralCheck.cpp
===================================================================
--- RawStringLiteralCheck.cpp
+++ RawStringLiteralCheck.cpp
@@ -38,7 +38,8 @@
   // Already a raw string literal if R comes before ".
   const size_t QuotePos = Text.find('"');
   assert(QuotePos != StringRef::npos);
-  return (QuotePos > 0) && (Text[QuotePos - 1] == 'R');
+  return (QuotePos != StringRef::npos) && (QuotePos > 0) &&
+         (Text[QuotePos - 1] == 'R');
 }
 
 bool containsEscapedCharacters(const MatchFinder::MatchResult &Result,