diff --git a/hphp/runtime/vm/translator/hopt/ir.h b/hphp/runtime/vm/translator/hopt/ir.h
index d9d225471..f3ffb0c8f 100644
--- a/hphp/runtime/vm/translator/hopt/ir.h
+++ b/hphp/runtime/vm/translator/hopt/ir.h
@@ -121,6 +121,7 @@ static const TCA kIRDirectGuardActive = (TCA)0x03;
  *     CStr          same as C(StaticStr)
  *     SNumInt       same as S(Int,Bool)
  *     SNum          same as S(Int,Bool,Dbl)
+ *     SSpills       SpillStack's variadic source list
  *
  * flags:
  *
@@ -350,7 +351,7 @@ O(StaticLocInitCached,    D(BoxedCell), CStr                                  \
                                           S(FramePtr)                         \
                                           S(Cell)                             \
                                             C(CacheHandle),      PRc|E|N|Mem) \
-O(SpillStack,                D(StkPtr), SUnk,                            CRc) \
+O(SpillStack,                D(StkPtr), S(StkPtr) C(Int) SSpills,        CRc) \
 O(SpillFrame,                D(StkPtr), S(StkPtr)                             \
                                           S(FramePtr)                         \
                                           S(Func,FuncCls,FuncCtx,Null)        \
diff --git a/hphp/runtime/vm/translator/hopt/type.cpp b/hphp/runtime/vm/translator/hopt/type.cpp
index b16edd022..e13a5b92c 100644
--- a/hphp/runtime/vm/translator/hopt/type.cpp
+++ b/hphp/runtime/vm/translator/hopt/type.cpp
@@ -215,6 +215,16 @@ void assertOperandTypes(const IRInstruction* inst) {
                        errorMessage).str());
   };
 
+  auto checkSpills = [&] {
+    for (; curSrc < inst->getNumSrcs(); ++curSrc) {
+      // SpillStack slots may be stack types or None, if the
+      // simplifier removed some.
+      auto const valid = inst->getSrc(curSrc)->getType()
+        .subtypeOfAny(Type::Gen, Type::Cls, Type::None);
+      check(valid, "Gen|Cls|None");
+    }
+  };
+
 #define IRT(name, ...) UNUSED static const Type name = Type::name;
   IR_TYPES
 #undef IRT
@@ -233,6 +243,7 @@ void assertOperandTypes(const IRInstruction* inst) {
 #define SNumInt  S(Int, Bool)
 #define SNum     S(Int, Bool, Dbl)
 #define SUnk     return;
+#define SSpills  checkSpills();
 #define ND
 #define DMulti
 #define DStk(...)
@@ -265,6 +276,7 @@ void assertOperandTypes(const IRInstruction* inst) {
 #undef CStr
 #undef SNum
 #undef SUnk
+#undef SSpills
 
 #undef ND
 #undef D