1 files changed, 1 insertions, 0 deletions
diff --git a/io.c b/io.c
index 75b7b4f34..2fc199d86 100644
--- a/io.c
+++ b/io.c
@@ -5621,6 +5621,7 @@ io_new_instance(VALUE args)
 static void
 io_encoding_set(rb_io_t *fptr, int argc, VALUE v1, VALUE v2)
 {
+    if (NIL_P(v2)) argc = 1;
     if (argc == 2) {
 	fptr->enc2 = rb_to_encoding(v1);
 	fptr->enc = rb_to_encoding(v2);