diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 4eb1511787..27c7f65408 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -915,9 +915,6 @@ strio_extend(struct StringIO *ptr, long pos, long len)
 	if (pos > olen)
 	    MEMZERO(RSTRING_PTR(ptr->string) + olen, char, pos - olen);
     }
-    else {
-	rb_str_modify(ptr->string);
-    }
 }
 
 /*
@@ -1464,14 +1461,9 @@ strio_write(VALUE self, VALUE str)
 	}
     }
     else {
-	int cr0 = ENC_CODERANGE(ptr->string);
-	int cr = ENC_CODERANGE_UNKNOWN;
-	if (rb_enc_asciicompat(enc) && rb_enc_asciicompat(enc2)) {
-	    cr = ENC_CODERANGE_AND(cr0, ENC_CODERANGE(str));
-	}
 	strio_extend(ptr, ptr->pos, len);
+	rb_str_modify(ptr->string);
 	memmove(RSTRING_PTR(ptr->string)+ptr->pos, RSTRING_PTR(str), len);
-	if (cr != cr0) ENC_CODERANGE_SET(ptr->string, cr);
     }
     RB_GC_GUARD(str);
     ptr->pos += len;
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 15fcc21146..8afbcf3545 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -961,6 +961,12 @@ class TestStringIO < Test::Unit::TestCase
     assert_predicate(s.string, :ascii_only?)
     s.write "\u{431 43e 433 443 441}"
     assert_not_predicate(s.string, :ascii_only?)
+
+    s = StringIO.new("\u{3042}")
+    s.rewind
+    assert_not_predicate(s.string, :ascii_only?)
+    s.write('aaaa')
+    assert_predicate(s.string, :ascii_only?)
   end
 
   def assert_string(content, encoding, str, mesg = nil)