diff --git a/ChangeLog b/ChangeLog
index 23806d8355..7d2e134a81 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Jun  8 21:57:41 2016  Martin Duerst  <duerst@it.aoyama.ac.jp>
+
+	* string.c: Special-case :ascii option in rb_str_upcase_bang.
+
 Wed Jun  8 21:28:36 2016  Martin Duerst  <duerst@it.aoyama.ac.jp>
 
 	* string.c: New static function rb_str_ascii_casemap; special-casing
diff --git a/string.c b/string.c
index e1cedf42aa..a8d06be52d 100644
--- a/string.c
+++ b/string.c
@@ -5871,7 +5871,6 @@ static VALUE
 rb_str_upcase_bang(int argc, VALUE *argv, VALUE str)
 {
     rb_encoding *enc;
-    int modify = 0;
     OnigCaseFoldType flags = ONIGENC_CASE_UPCASE;
 
     flags = check_case_options(argc, argv, flags);
@@ -5887,17 +5886,17 @@ rb_str_upcase_bang(int argc, VALUE *argv, VALUE str)
 
 	    if (rb_enc_isascii(c, enc) && 'a' <= c && c <= 'z') {
 		*s = 'A' + (c - 'a');
-		modify = 1;
+		flags |= ONIGENC_CASE_MODIFIED;
 	    }
 	    s++;
 	}
     }
-    else {
+    else if (flags&ONIGENC_CASE_ASCII_ONLY)
+        rb_str_ascii_casemap(str, &flags, enc);
+    else
 	str_shared_replace(str, rb_str_casemap(str, &flags, enc));
-	modify = ONIGENC_CASE_MODIFIED & flags;
-    }
 
-    if (modify) return str;
+    if (ONIGENC_CASE_MODIFIED&flags) return str;
     return Qnil;
 }