diff --git a/ChangeLog b/ChangeLog
index b8291dc4cf..80406bf540 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Aug 20 10:43:24 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/stringio/stringio.c (strio_read): return new string if nil
+	  is explicitly given as a buffer ([Bug #5207]), otherwise set the
+	  encoding.  also removed dead code.
+
 Fri Aug 19 14:25:51 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* process.c (proc_spawn_v, proc_spawn): should not wait the
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 68baf350f5..5aa2d27ef3 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -1219,12 +1219,15 @@ strio_read(int argc, VALUE *argv, VALUE self)
     struct StringIO *ptr = readable(StringIO(self));
     VALUE str = Qnil;
     long len;
+    int binary = 0;
 
     switch (argc) {
       case 2:
 	str = argv[1];
-	StringValue(str);
-	rb_str_modify(str);
+	if (!NIL_P(str)) {
+	    StringValue(str);
+	    rb_str_modify(str);
+	}
       case 1:
 	if (!NIL_P(argv[0])) {
 	    len = NUM2LONG(argv[0]);
@@ -1235,6 +1238,7 @@ strio_read(int argc, VALUE *argv, VALUE self)
 		if (!NIL_P(str)) rb_str_resize(str, 0);
 		return Qnil;
 	    }
+	    binary = 1;
 	    break;
 	}
 	/* fall through */
@@ -1258,21 +1262,19 @@ strio_read(int argc, VALUE *argv, VALUE self)
     }
     if (NIL_P(str)) {
 	str = strio_substr(ptr, ptr->pos, len);
-	if (argc > 0) rb_enc_associate(str, rb_ascii8bit_encoding());
+	if (binary) rb_enc_associate(str, rb_ascii8bit_encoding());
     }
     else {
 	long rest = RSTRING_LEN(ptr->string) - ptr->pos;
 	if (len > rest) len = rest;
 	rb_str_resize(str, len);
 	MEMCPY(RSTRING_PTR(str), RSTRING_PTR(ptr->string) + ptr->pos, char, len);
+	if (binary)
+	    rb_enc_associate(str, rb_ascii8bit_encoding());
+	else
+	    rb_enc_copy(str, ptr->string);
     }
-    if (NIL_P(str)) {
-	str = rb_str_new(0, 0);
-	len = 0;
-    }
-    else {
-	ptr->pos += len = RSTRING_LEN(str);
-    }
+    ptr->pos += RSTRING_LEN(str);
     return str;
 }
 
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 0258218755..26fcc624e2 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -418,6 +418,14 @@ class TestStringIO < Test::Unit::TestCase
     assert_equal("\u3042\u3044", f.read)
     f.rewind
     assert_equal("\u3042\u3044".force_encoding(Encoding::ASCII_8BIT), f.read(f.size))
+
+    bug5207 = '[ruby-core:39026]'
+    f.rewind
+    assert_equal("\u3042\u3044", f.read(nil, nil), bug5207)
+    f.rewind
+    s = ""
+    f.read(nil, s)
+    assert_equal("\u3042\u3044", s, bug5207)
   end
 
   def test_readpartial