ReceiverCheckMode needs to be utilized further.

src/arm64/code-stubs-arm64.cc

Index: src/arm64/code-stubs-arm64.cc
diff --git a/src/arm64/code-stubs-arm64.cc b/src/arm64/code-stubs-arm64.cc
index 9056772324e5fb98606d5f4d1be30efbe3e4b369..ed951ae7eb925074a9a9aacaa4bc2e1d49aa7a65 100644
--- a/src/arm64/code-stubs-arm64.cc
+++ b/src/arm64/code-stubs-arm64.cc
@@ -3812,9 +3812,9 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // x12: input_type
   // x15: from (untagged)
   __ SmiTag(from);
-  StringCharAtGenerator generator(
-      input_string, from, result_length, x0,
-      &runtime, &runtime, &runtime, STRING_INDEX_IS_NUMBER);
+  StringCharAtGenerator generator(input_string, from, result_length, x0,
+                                  &runtime, &runtime, &runtime,
+                                  STRING_INDEX_IS_NUMBER, RECEIVER_IS_STRING);
   generator.GenerateFast(masm);
   __ Drop(3);
   __ Ret();