diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 74e2b95..27c7f65 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);
-    }
 }
 
 /*
@@ -1465,6 +1462,7 @@ strio_write(VALUE self, VALUE str)
     }
     else {
 	strio_extend(ptr, ptr->pos, len);
+	rb_str_modify(ptr->string);
 	memmove(RSTRING_PTR(ptr->string)+ptr->pos, RSTRING_PTR(str), len);
     }
     RB_GC_GUARD(str);
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 216b06d..a5c1ff7 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -922,6 +922,16 @@ def test_encoding_write
     assert_equal("abc".encode("utf-32be"), s.string)
   end
 
+  def test_write_str_modify
+    seek_pos = [0, 2, 4, 6]
+    seek_pos.each do |pos|
+      s = StringIO.new('abcd')
+      s.seek(pos)
+      s.write("\xff\xff\xff")
+      refute(s.string.ascii_only?)
+    end
+  end
+
   def test_encoding_read
     s = StringIO.new("abc".encode("utf-32be"), "r:utf-8")
     assert_equal("\0\0\0a\0\0\0b\0\0\0c", s.read)