diff --git a/src/string.c b/src/string.c
index ed58c484b..856e908af 100644
--- a/src/string.c
+++ b/src/string.c
@@ -973,6 +973,8 @@ mrb_str_to_str(mrb_state *mrb, mrb_value str)
   switch (mrb_type(str)) {
   case MRB_TT_STRING:
     return str;
+  case MRB_TT_SYMBOL:
+    return mrb_sym2str(mrb, mrb_symbol(str));
   case MRB_TT_FIXNUM:
     return mrb_fixnum_to_str(mrb, str, 10);
   case MRB_TT_CLASS: