diff --git a/Lex/HeaderSearch.cpp b/Lex/HeaderSearch.cpp
index e4f7aeb..bd36f35 100644
--- a/Lex/HeaderSearch.cpp
+++ b/Lex/HeaderSearch.cpp
@@ -293,6 +293,8 @@
 LookupSubframeworkHeader(const char *FilenameStart,
                          const char *FilenameEnd,
                          const FileEntry *ContextFileEnt) {
+  assert(ContextFileEnt && "No context file?");
+  
   // Framework names must have a '/' in the filename.  Find it.
   const char *SlashPos = std::find(FilenameStart, FilenameEnd, '/');
   if (SlashPos == FilenameEnd) return 0;