diff --git a/string.c b/string.c
index b8ff2dd8bf..2febe52929 100644
--- a/string.c
+++ b/string.c
@@ -6600,7 +6600,7 @@ rb_str_upcase_bang(int argc, VALUE *argv, VALUE str)
     str_modify_keep_cr(str);
     enc = STR_ENC_GET(str);
     rb_str_check_dummy_enc(enc);
-    if (((flags&ONIGENC_CASE_ASCII_ONLY) && (enc==rb_utf8_encoding() || rb_enc_mbmaxlen(enc)==1))
+    if (((flags&ONIGENC_CASE_ASCII_ONLY) && (rb_enc_mbmaxlen(enc)==1))
 	|| (!(flags&ONIGENC_CASE_FOLD_TURKISH_AZERI) && ENC_CODERANGE(str)==ENC_CODERANGE_7BIT)) {
         char *s = RSTRING_PTR(str), *send = RSTRING_END(str);
 
@@ -6666,7 +6666,7 @@ rb_str_downcase_bang(int argc, VALUE *argv, VALUE str)
     str_modify_keep_cr(str);
     enc = STR_ENC_GET(str);
     rb_str_check_dummy_enc(enc);
-    if (((flags&ONIGENC_CASE_ASCII_ONLY) && (enc==rb_utf8_encoding() || rb_enc_mbmaxlen(enc)==1))
+    if (((flags&ONIGENC_CASE_ASCII_ONLY) && (rb_enc_mbmaxlen(enc)==1))
 	|| (!(flags&ONIGENC_CASE_FOLD_TURKISH_AZERI) && ENC_CODERANGE(str)==ENC_CODERANGE_7BIT)) {
         char *s = RSTRING_PTR(str), *send = RSTRING_END(str);