diff --git i/eval.c w/eval.c
index f0a06fb..fbe17d0 100644
--- i/eval.c
+++ w/eval.c
@@ -52,7 +52,6 @@ ruby_setup(void)
     Init_BareVM();
     Init_heap();
     Init_vm_objects();
-    Init_frozen_strings();
 
     PUSH_TAG();
     if ((state = EXEC_TAG()) == 0) {
diff --git i/string.c w/string.c
index e7a971b..758854b 100644
--- i/string.c
+++ w/string.c
@@ -174,74 +174,6 @@ mustnot_broken(VALUE str)
     }
 }
 
-static int fstring_cmp(VALUE a, VALUE b);
-
-/* in case we restart MVM development, this needs to be per-VM */
-static st_table* frozen_strings;
-
-static inline st_table*
-rb_vm_fstring_table(void)
-{
-    return frozen_strings;
-}
-
-static const struct st_hash_type fstring_hash_type = {
-    fstring_cmp,
-    rb_str_hash,
-};
-
-static int
-fstr_update_callback(st_data_t *key, st_data_t *value, st_data_t arg, int existing)
-{
-    VALUE *fstr = (VALUE *)arg;
-    VALUE str = (VALUE)*key;
-
-    if (existing) {
-	/* because of lazy sweep, str may be unmarked already and swept
-	 * at next time */
-
-	if (rb_objspace_garbage_object_p(str)) {
-	    *fstr = Qundef;
-	    return ST_DELETE;
-	}
-
-	*fstr = str;
-	return ST_STOP;
-    }
-    else {
-	if (STR_SHARED_P(str)) { /* str should not be shared */
-	    str = rb_enc_str_new(RSTRING_PTR(str), RSTRING_LEN(str), STR_ENC_GET(str));
-	    OBJ_FREEZE(str);
-	}
-	else {
-	    str = rb_str_new_frozen(str);
-	}
-	RBASIC(str)->flags |= RSTRING_FSTR;
-
-	*key = *value = *fstr = str;
-	return ST_CONTINUE;
-    }
-}
-
-VALUE
-rb_fstring(VALUE str)
-{
-    VALUE ret;
-
-    Check_Type(str, T_STRING);
-
-    if (FL_TEST(str, RSTRING_FSTR))
-	return str;
-
-    do {
-	ret = str;
-	st_update(rb_vm_fstring_table(), (st_data_t)str,
-		    fstr_update_callback, (st_data_t)&ret);
-    } while (ret == Qundef);
-
-    return ret;
-}
-
 static VALUE
 setup_fake_str(struct RString *fake_str, const char *name, long len, int encidx)
 {
@@ -263,30 +195,6 @@ rb_setup_fake_str(struct RString *fake_str, const char *name, long len, rb_encod
     return setup_fake_str(fake_str, name, len, rb_enc_to_index(enc));
 }
 
-VALUE
-rb_fstring_new(const char *ptr, long len)
-{
-    struct RString fake_str;
-    return rb_fstring(setup_fake_str(&fake_str, ptr, len, ENCINDEX_US_ASCII));
-}
-
-static int
-fstring_set_class_i(st_data_t key, st_data_t val, st_data_t arg)
-{
-    RBASIC_SET_CLASS((VALUE)key, (VALUE)arg);
-    return ST_CONTINUE;
-}
-
-static int
-fstring_cmp(VALUE a, VALUE b)
-{
-    int cmp = rb_str_hash_cmp(a, b);
-    if (cmp != 0) {
-	return cmp;
-    }
-    return ENCODING_GET(b) - ENCODING_GET(a);
-}
-
 static inline int
 single_byte_optimizable(VALUE str)
 {
@@ -956,8 +864,8 @@ void
 rb_str_free(VALUE str)
 {
     if (FL_TEST(str, RSTRING_FSTR)) {
-	st_data_t fstr = (st_data_t)str;
-	st_delete(rb_vm_fstring_table(), &fstr, NULL);
+	void rb_fstring_remove(VALUE str);
+	rb_fstring_remove(str);
     }
 
     if (!STR_EMBED_P(str) && !FL_TEST(str, STR_SHARED)) {
@@ -8955,13 +8863,5 @@ Init_String(void)
 
     rb_define_method(rb_cSymbol, "encoding", sym_encoding, 0);
 
-    assert(rb_vm_fstring_table());
-    st_foreach(rb_vm_fstring_table(), fstring_set_class_i, rb_cString);
-}
-
-void
-Init_frozen_strings(void)
-{
-    assert(!frozen_strings);
-    frozen_strings = st_init_table(&fstring_hash_type);
+    Init_frozen_strings();
 }
diff --git i/vm.c w/vm.c
index cd80729..78d640e 100644
--- i/vm.c
+++ w/vm.c
@@ -2787,6 +2787,108 @@ Init_BareVM(void)
     ruby_thread_init_stack(th);
 }
 
+static inline st_table *
+rb_vm_fstring_table(void)
+{
+    return GET_VM()->frozen_strings;
+}
+
+static int
+fstring_cmp(VALUE a, VALUE b)
+{
+    int cmp = rb_str_hash_cmp(a, b);
+    if (cmp != 0) {
+	return cmp;
+    }
+    return ENCODING_GET(b) - ENCODING_GET(a);
+}
+
+static const struct st_hash_type fstring_hash_type = {
+    fstring_cmp,
+    rb_str_hash,
+};
+
+static int
+fstr_update_callback(st_data_t *key, st_data_t *value, st_data_t arg, int existing)
+{
+    VALUE *fstr = (VALUE *)arg;
+    VALUE str = (VALUE)*key;
+
+    if (existing) {
+	/* because of lazy sweep, str may be unmarked already and swept
+	 * at next time */
+
+	if (rb_objspace_garbage_object_p(str)) {
+	    *fstr = Qundef;
+	    return ST_DELETE;
+	}
+
+	*fstr = str;
+	return ST_STOP;
+    }
+    else {
+	if (STR_SHARED_P(str)) { /* str should not be shared */
+	    str = rb_enc_str_new(RSTRING_PTR(str), RSTRING_LEN(str), rb_enc_get(str));
+	    OBJ_FREEZE(str);
+	}
+	else {
+	    str = rb_str_new_frozen(str);
+	}
+	RBASIC(str)->flags |= RSTRING_FSTR;
+
+	*key = *value = *fstr = str;
+	return ST_CONTINUE;
+    }
+}
+
+VALUE
+rb_fstring(VALUE str)
+{
+    VALUE ret;
+    st_table *frozen_strings = rb_vm_fstring_table();
+
+    Check_Type(str, T_STRING);
+
+    if (FL_TEST(str, RSTRING_FSTR))
+	return str;
+
+    do {
+	ret = str;
+	st_update(frozen_strings, (st_data_t)str, fstr_update_callback, (st_data_t)&ret);
+    } while (ret == Qundef);
+
+    return ret;
+}
+
+VALUE
+rb_fstring_new(const char *ptr, long len)
+{
+    struct RString fake_str;
+    return rb_fstring(rb_setup_fake_str(&fake_str, ptr, len, rb_usascii_encoding()));
+}
+
+void
+rb_fstring_remove(VALUE str)
+{
+    st_data_t fstr = (st_data_t)str;
+    st_delete(rb_vm_fstring_table(), &fstr, NULL);
+}
+
+static int
+fstring_set_class_i(st_data_t key, st_data_t val, st_data_t arg)
+{
+    RBASIC_SET_CLASS((VALUE)key, (VALUE)arg);
+    return ST_CONTINUE;
+}
+
+void
+Init_frozen_strings(void)
+{
+    st_table *frozen_strings = rb_vm_fstring_table();
+    assert(frozen_strings);
+    st_foreach(frozen_strings, fstring_set_class_i, rb_cString);
+}
+
 void
 Init_vm_objects(void)
 {
@@ -2796,6 +2898,9 @@ Init_vm_objects(void)
 
     /* initialize mark object array, hash */
     vm->mark_object_ary = rb_ary_tmp_new(128);
+
+    assert(!vm->frozen_strings);
+    vm->frozen_strings = st_init_table(&fstring_hash_type);
 }
 
 /* top self */