2 files changed, 5 insertions, 2 deletions
diff --git a/src/ir/cost.h b/src/ir/cost.h
index 8ebc6eb9b..b48b777e1 100644
--- a/src/ir/cost.h
+++ b/src/ir/cost.h
@@ -672,7 +672,7 @@ struct CostAnalyzer : public OverriddenVisitor<CostAnalyzer, CostType> {
   }
   CostType visitRefAs(RefAs* curr) { return 1 + visit(curr->value); }
   CostType visitStringNew(StringNew* curr) {
-    return 8 + visit(curr->ptr) + visit(curr->length);
+    return 8 + visit(curr->ptr) + maybeVisit(curr->length);
   }
   CostType visitStringConst(StringConst* curr) { return 4; }
   CostType visitStringMeasure(StringMeasure* curr) {
diff --git a/src/ir/effects.h b/src/ir/effects.h
index 58480f0ff..43d4868d8 100644
--- a/src/ir/effects.h
+++ b/src/ir/effects.h
@@ -732,7 +732,10 @@ private:
       // we keep the code here simpler, but it does mean another optimization
       // cycle may be needed in some cases.
     }
-    void visitStringNew(StringNew* curr) {}
+    void visitStringNew(StringNew* curr) {
+      // traps when out of bounds in linear memory or ref is null
+      parent.implicitTrap = true;
+    }
     void visitStringConst(StringConst* curr) {}
     void visitStringMeasure(StringMeasure* curr) {
       // traps when ref is null.