ReceiverCheckMode needs to be utilized further.

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 8e6fc4496a09bdb371fecac9dbfbe09a438cb101..0f26d3ba5a7c1b63d497f55e722e4546ee56071d 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -3106,8 +3106,9 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // 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);