diff --git a/lib/AST/Expr.cpp b/lib/AST/Expr.cpp
index 427c331..96a8125 100644
--- a/lib/AST/Expr.cpp
+++ b/lib/AST/Expr.cpp
@@ -1394,6 +1394,7 @@
 }
 
 void InitListExpr::setArrayFiller(Expr *filler) {
+  assert(!hasArrayFiller() && "Filler already set!");
   ArrayFillerOrUnionFieldInit = filler;
   // Fill out any "holes" in the array due to designated initializers.
   Expr **inits = getInits();