diff --git a/core/io/shared/new.rb b/core/io/shared/new.rb
index cba5f33eb..01e08f511 100644
--- a/core/io/shared/new.rb
+++ b/core/io/shared/new.rb
@@ -324,6 +324,9 @@
     -> {
       @io = IO.send(@method, @fd, 'w:ISO-8859-1', external_encoding: 'ISO-8859-1')
     }.should raise_error(ArgumentError)
+    -> {
+      @io = IO.send(@method, @fd, 'w:ISO-8859-1', internal_encoding: 'ISO-8859-1')
+    }.should raise_error(ArgumentError)
     -> {
       @io = IO.send(@method, @fd, 'w:ISO-8859-1:UTF-8', internal_encoding: 'ISO-8859-1')
     }.should raise_error(ArgumentError)