diff --git a/src/asm_x64.c b/src/asm_x64.c
index 39658d4..0d25af1 100644
--- a/src/asm_x64.c
+++ b/src/asm_x64.c
@@ -304,6 +304,7 @@ static const uint8_t call_regs[] = {RDI, RSI, RDX, RCX, R8, R9};
 
 static void assign_call_registers(uint16_t op, trace_s *trace, int *slot,
                                   uint32_t *next_spill, int arg) {
+  assert(arg < 6); // all in reg
   if (!ir_is_const(op)) {
     auto cop = &trace->ops[op];
     if (cop->op == IR_CARG) {
diff --git a/src/record.c b/src/record.c
index 52137e3..391f80c 100644
--- a/src/record.c
+++ b/src/record.c
@@ -1584,6 +1584,23 @@ bool record_instr(uint32_t *pc, gc_obj *frame, int64_t argcnt) {
     // No stack top tracking
     break;
   }
+  case STRING_COPY: {
+    auto slot = INS_A(i);
+    auto arg4 = push_ir(trace, IR_CARG, record_stack_load(slot+3, frame),
+                       record_stack_load(slot+4, frame), UNDEFINED_TAG);
+    auto arg3 = push_ir(trace, IR_CARG, 
+			record_stack_load(slot+2, frame), arg4, UNDEFINED_TAG);
+    auto arg2 = push_ir(trace, IR_CARG, 
+			record_stack_load(slot+1, frame), arg3, UNDEFINED_TAG);
+    auto arg1 = push_ir(trace, IR_CARG, 
+			record_stack_load(slot, frame), arg2, UNDEFINED_TAG);
+    auto knum = arrlen(trace->consts);
+    arrput(trace->consts, tag_ptr(vm_string_copy));
+    push_ir(trace, IR_CALLXS, arg1, knum | IR_CONST_BIAS, UNDEFINED_TAG);
+    stack_top = INS_A(i);
+    add_snap(regs_list, regs - regs_list - 1, trace, pc + 1, depth, stack_top);
+    break;
+  }
   case READ: {
     port_s *port = to_port(frame[INS_B(i)]);
     uint8_t type = CHAR_TAG;
diff --git a/src/vm.c b/src/vm.c
index 919c3b0..9f3fb84 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -811,9 +811,8 @@ LIBRARY_FUNC_B(CLOSURE) {
   auto fun = to_func(frame[ra]);
   if (fun->poly_cnt < 50) {
     if (fun->poly_cnt == 1) {
-      printf("Make polymorphic %s\n", fun->name);
       for (uint32_t i = 0; i < hmlen(fun->lst); i++) {
-	printf("POLYMORPHIC %s flush of trace %i\n", fun->name, fun->lst[i].key);
+	//printf("POLYMORPHIC %s flush of trace %i\n", fun->name, fun->lst[i].key);
         trace_flush(trace_cache_get(fun->lst[i].key), true);
       }
       hmfree(fun->lst);
@@ -1222,6 +1221,12 @@ LIBRARY_FUNC_BC_LOAD_NAME("STRING-SET!", STRING_SET) {
 }
 END_LIBRARY_FUNC
 
+void vm_string_copy(gc_obj tostr, gc_obj tostart, gc_obj fromstr, gc_obj fromstart, gc_obj fromend) {
+  auto len = to_fixnum(fromend) - to_fixnum(fromstart);
+  memcpy(&to_string(tostr)->str[to_fixnum(tostart)],
+	 &to_string(fromstr)->str[to_fixnum(fromstart)], len);
+}
+
 LIBRARY_FUNC_NAME("STRING-COPY", STRING_COPY) {
   // TODO(djwatson) Some of this is already checked in bootstrap?
   LOAD_TYPE_WITH_CHECK(tostr, string_s, frame[ra], STRING_TAG);
diff --git a/src/vm.h b/src/vm.h
index ac88e48..02208ba 100644
--- a/src/vm.h
+++ b/src/vm.h
@@ -32,6 +32,7 @@ gc_obj vm_length(gc_obj fb);
 gc_obj vm_memq(gc_obj fb, gc_obj fc);
 gc_obj vm_assq(gc_obj fb, gc_obj fc);
 gc_obj vm_assv(gc_obj fb, gc_obj fc);
+void vm_string_copy(gc_obj tostr, gc_obj tostart, gc_obj fromstr, gc_obj fromstart, gc_obj fromend);
 
 static inline uint32_t hotmap_hash(const uint32_t *pc) {
   return (((uint64_t)pc) >> 2) & hotmap_mask;