diff --git a/ChangeLog b/ChangeLog
index acb563623b..dab168c311 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Aug 29 02:11:46 2008  Tanaka Akira  <akr@fsij.org>
+
+	* transcode.c (econv_primitive_convert): accept nil as input for empty
+	  input.
+
 Fri Aug 29 02:03:56 2008  Shugo Maeda  <shugo@ruby-lang.org>
 
 	* strftime.c (rb_strftime): supported %s and %P.
diff --git a/test/ruby/test_econv.rb b/test/ruby/test_econv.rb
index 5324e6dfe2..bc22da567d 100644
--- a/test/ruby/test_econv.rb
+++ b/test/ruby/test_econv.rb
@@ -79,6 +79,12 @@ class TestEncodingConverter < Test::Unit::TestCase
     }
   end
 
+  def test_nil_input
+    ec = Encoding::Converter.new("UTF-8", "EUC-JP")
+    ret = ec.primitive_convert(nil, dst="", nil, 10)
+    assert_equal(:finished, ret)
+  end
+
   def test_partial_input
     ec = Encoding::Converter.new("UTF-8", "EUC-JP")
     ret = ec.primitive_convert(src="", dst="", nil, 10, Encoding::Converter::PARTIAL_INPUT)
diff --git a/transcode.c b/transcode.c
index 861c285eed..c6f7b73e6a 100644
--- a/transcode.c
+++ b/transcode.c
@@ -2339,8 +2339,13 @@ econv_result_to_symbol(rb_econv_result_t res)
  *
  * primitive_convert converts source_buffer into destination_buffer.
  *
- * source_buffer and destination_buffer should be a string.
+ * source_buffer should be a string or nil.
+ * nil means a empty string.
+ *
+ * adestination_buffer should be a string.
+ *
  * destination_byteoffset should be an integer or nil.
+ *
  * destination_bytesize and flags should be an integer.
  *
  * primitive_convert convert the content of source_buffer from beginning
@@ -2415,7 +2420,8 @@ econv_primitive_convert(int argc, VALUE *argv, VALUE self)
         flags = NUM2INT(flags_v);
 
     StringValue(output);
-    StringValue(input);
+    if (!NIL_P(input))
+        StringValue(input);
     rb_str_modify(output);
 
     if (output_byteoffset_v == Qnil)
@@ -2440,15 +2446,21 @@ econv_primitive_convert(int argc, VALUE *argv, VALUE self)
     if (rb_str_capacity(output) < output_byteend)
         rb_str_resize(output, output_byteend);
 
-    ip = (const unsigned char *)RSTRING_PTR(input);
-    is = ip + RSTRING_LEN(input);
+    if (NIL_P(input)) {
+        ip = is = NULL;
+    }
+    else {
+        ip = (const unsigned char *)RSTRING_PTR(input);
+        is = ip + RSTRING_LEN(input);
+    }
 
     op = (unsigned char *)RSTRING_PTR(output) + output_byteoffset;
     os = op + output_bytesize;
 
     res = rb_econv_convert(ec, &ip, is, &op, os, flags);
     rb_str_set_len(output, op-(unsigned char *)RSTRING_PTR(output));
-    rb_str_drop_bytes(input, ip - (unsigned char *)RSTRING_PTR(input));
+    if (!NIL_P(input))
+        rb_str_drop_bytes(input, ip - (unsigned char *)RSTRING_PTR(input));
 
     if (ec->destination_encoding) {
         rb_enc_associate(output, ec->destination_encoding);