diff --git a/hphp/runtime/vm/translator/hopt/vectortranslator.cpp b/hphp/runtime/vm/translator/hopt/vectortranslator.cpp
index 719a18675..1a01967d3 100644
--- a/hphp/runtime/vm/translator/hopt/vectortranslator.cpp
+++ b/hphp/runtime/vm/translator/hopt/vectortranslator.cpp
@@ -167,6 +167,12 @@ void VectorEffects::init(Opcode op, const Type origBase,
     valType = definitelyFail ? Type::InitNull : (valType | Type::InitNull);
   }
 
+  if (op == SetElem && baseType.maybe(Type::Str)) {
+    // If the base is a String, the result will be different from the value
+    // use valTypeChanged (even though the type may not have)
+    valTypeChanged = true;
+  }
+
   // The final baseType should be a pointer/box iff the input was
   baseType = baseBoxed ? baseType.box() : baseType;
   baseType = basePtr   ? baseType.ptr() : baseType;
diff --git a/hphp/test/vm/setmstr2.php b/hphp/test/vm/setmstr2.php
new file mode 100644
index 000000000..24b5cdae7
--- /dev/null
+++ b/hphp/test/vm/setmstr2.php
@@ -0,0 +1,10 @@
+<?php
+function bar($a) {}
+function baz() {
+  $cipher = 'abcdefghij';
+  $pos = 4;
+  $random_byte = chr(25);
+  $cipher[$pos] = $random_byte;
+  var_dump(ord($random_byte));
+}
+baz();
diff --git a/hphp/test/vm/setmstr2.php.exp b/hphp/test/vm/setmstr2.php.exp
new file mode 100644
index 000000000..a43ede577
--- /dev/null
+++ b/hphp/test/vm/setmstr2.php.exp
@@ -0,0 +1 @@
+int(25)