--- a/js/src/jit/IonBuilder.cpp
+++ b/js/src/jit/IonBuilder.cpp
@@ -8563,19 +8563,22 @@ AbortReasonOr<Ok> IonBuilder::setStaticN
   // Don't optimize global lexical bindings if they aren't initialized at
   // compile time.
   if (isGlobalLexical && IsUninitializedGlobalLexicalSlot(staticObject, name)) {
     return jsop_setprop(name);
   }
 
   current->pop();
 
-  // Pop the bound object on the stack.
+  // Pop the bound object on the stack. This is usually a constant but it can
+  // be a phi if loops are involved, for example: x = [...arr];
   MDefinition* obj = current->pop();
-  MOZ_ASSERT(&obj->toConstant()->toObject() == staticObject);
+  MOZ_ASSERT(obj->isConstant() || obj->isPhi());
+  MOZ_ASSERT_IF(obj->isConstant(),
+                &obj->toConstant()->toObject() == staticObject);
 
   if (needsPostBarrier(value)) {
     current->add(MPostWriteBarrier::New(alloc(), obj, value));
   }
 
   // If the property has a known type, we may be able to optimize typed stores
   // by not storing the type tag.
   MIRType slotType = MIRType::None;