diff --git a/lib/CodeGen/LiveInterval.cpp b/lib/CodeGen/LiveInterval.cpp
index e18d7c7..0ea8f59 100644
--- a/lib/CodeGen/LiveInterval.cpp
+++ b/lib/CodeGen/LiveInterval.cpp
@@ -40,6 +40,7 @@
 }
 
 LiveInterval::iterator LiveInterval::find(SlotIndex Pos) {
+  assert(Pos.isValid() && "Cannot search for an invalid index");
   return std::upper_bound(begin(), end(), LiveRange(SlotIndex(), Pos, 0),
                           CompEnd());
 }