diff --git a/lib/Basic/SourceManager.cpp b/lib/Basic/SourceManager.cpp
index 0c00865..f1965e7 100644
--- a/lib/Basic/SourceManager.cpp
+++ b/lib/Basic/SourceManager.cpp
@@ -1032,7 +1032,8 @@
   // See if we just calculated the line number for this FilePos and can use
   // that to lookup the start of the line instead of searching for it.
   if (LastLineNoFileIDQuery == FID &&
-      LastLineNoContentCache->SourceLineCache != 0) {
+      LastLineNoContentCache->SourceLineCache != 0 &&
+      LastLineNoResult < LastLineNoContentCache->NumLines) {
     unsigned *SourceLineCache = LastLineNoContentCache->SourceLineCache;
     unsigned LineStart = SourceLineCache[LastLineNoResult - 1];
     unsigned LineEnd = SourceLineCache[LastLineNoResult];