diff --git a/src/io.c b/src/io.c
index 51a659f0e..7a1121338 100644
--- a/src/io.c
+++ b/src/io.c
@@ -623,7 +623,10 @@ mrb_io_sysread(mrb_state *mrb, mrb_value io)
 
   mrb_get_args(mrb, "i|S", &maxlen, &buf);
   if (maxlen < 0) {
-    return mrb_nil_value();
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "negative expanding string size");
+  }
+  else if (maxlen == 0) {
+    return mrb_str_new(mrb, NULL, maxlen);
   }
 
   if (mrb_nil_p(buf)) {
@@ -633,7 +636,7 @@ mrb_io_sysread(mrb_state *mrb, mrb_value io)
     buf = mrb_str_resize(mrb, buf, maxlen);
   }
 
-  fptr = (struct mrb_io *)mrb_get_datatype(mrb, io, &mrb_io_type);
+  fptr = (struct mrb_io *)io_get_open_fptr(mrb, io);
   ret = read(fptr->fd, RSTRING_PTR(buf), maxlen);
   switch (ret) {
     case 0: /* EOF */
diff --git a/test/io.rb b/test/io.rb
index 955481b9f..9ce8985f7 100644
--- a/test/io.rb
+++ b/test/io.rb
@@ -234,6 +234,9 @@ assert('IO.sysopen, IO#sysread') do
     io.sysread(10000)
   end
   io.close
+  assert_equal "", io.sysread(0)
+  assert_raise(IOError) { io.sysread(1) }
+  assert_raise(ArgumentError) { io.sysread(-1) }
   io.closed?
 end