1 files changed, 1 insertions, 1 deletions
diff --git a/mrblib/io.rb b/mrblib/io.rb
index 4c69a4b99..249488ac4 100644
--- a/mrblib/io.rb
+++ b/mrblib/io.rb
@@ -99,7 +99,7 @@ class IO
     return str.size unless str.size > 0
 
     len = syswrite(str)
-    if str.size == len
+    if len != -1
       @pos += len
       return len
     end