diff --git a/string.c b/string.c
index 34c77a5054..6ef92eb9fb 100644
--- a/string.c
+++ b/string.c
@@ -212,6 +212,7 @@ 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 VALUE register_fstring(VALUE str);
 
 static inline st_table*
 rb_vm_fstring_table(void)
@@ -260,13 +261,19 @@ fstr_update_callback(st_data_t *key, st_data_t *value, st_data_t arg, int existi
 VALUE
 rb_fstring(VALUE str)
 {
-    VALUE ret;
-
     Check_Type(str, T_STRING);
 
     if (FL_TEST(str, RSTRING_FSTR))
 	return str;
 
+    return register_fstring(str);
+}
+
+static VALUE
+register_fstring(VALUE str)
+{
+    VALUE ret;
+
     do {
 	ret = str;
 	st_update(rb_vm_fstring_table(), (st_data_t)str,
@@ -301,7 +308,7 @@ 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));
+    return register_fstring(setup_fake_str(&fake_str, ptr, len, ENCINDEX_US_ASCII));
 }
 
 VALUE