ReceiverCheckMode needs to be utilized further.

src/x64/code-stubs-x64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/bootstrapper.h"
 #include "src/code-stubs.h"
@@ skipping 3088 matching lines @@
 
   // Just jump to runtime to create the sub string.
   __ bind(&runtime);
   __ TailCallRuntime(Runtime::kSubString, 3, 1);
 
   __ bind(&single_char);
   // rax: string
   // rbx: instance type
   // rcx: sub string length (smi)
   // rdx: from index (smi)
-  StringCharAtGenerator generator(
-      rax, rdx, rcx, rax, &runtime, &runtime, &runtime, STRING_INDEX_IS_NUMBER);
+  StringCharAtGenerator generator(rax, rdx, rcx, rax, &runtime, &runtime,
+                                  &runtime, STRING_INDEX_IS_NUMBER,
+                                  RECEIVER_IS_STRING);
   generator.GenerateFast(masm);
   __ ret(SUB_STRING_ARGUMENT_COUNT * kPointerSize);
   generator.SkipSlow(masm, &runtime);
 }
 
 
 void StringHelper::GenerateFlatOneByteStringEquals(MacroAssembler* masm,
                                                    Register left,
                                                    Register right,
                                                    Register scratch1,
@@ skipping 1578 matching lines @@
                               return_value_operand,
                               NULL);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64