ReceiverCheckMode needs to be utilized further.

src/mips/code-stubs-mips.cc

Index: src/mips/code-stubs-mips.cc
diff --git a/src/mips/code-stubs-mips.cc b/src/mips/code-stubs-mips.cc
index 4eb11942a5efb6a952d9e89a4d64eb11cf948fab..de834a7d6588f545c213b531e6e6730c140e230c 100644
--- a/src/mips/code-stubs-mips.cc
+++ b/src/mips/code-stubs-mips.cc
@@ -3334,8 +3334,8 @@ void SubStringStub::Generate(MacroAssembler* masm) {
   // a2: length
   // a3: from index (untagged)
   __ SmiTag(a3, a3);
-  StringCharAtGenerator generator(
-      v0, a3, a2, v0, &runtime, &runtime, &runtime, STRING_INDEX_IS_NUMBER);
+  StringCharAtGenerator generator(v0, a3, a2, v0, &runtime, &runtime, &runtime,
+                                  STRING_INDEX_IS_NUMBER, RECEIVER_IS_STRING);
   generator.GenerateFast(masm);
   __ DropAndRet(3);
   generator.SkipSlow(masm, &runtime);