diff --git a/spec/ruby/optional/capi/ext/io_spec.c b/spec/ruby/optional/capi/ext/io_spec.c
index 2f86bf9f62..224b1fe513 100644
--- a/spec/ruby/optional/capi/ext/io_spec.c
+++ b/spec/ruby/optional/capi/ext/io_spec.c
@@ -150,6 +150,7 @@ VALUE io_spec_rb_io_wait_readable(VALUE self, VALUE io, VALUE read_p) {
   if (set_non_blocking(fd) == -1)
     rb_sys_fail("set_non_blocking failed");
 
+# if !SET_NON_BLOCKING_FAILS_ALWAYS
   if(RTEST(read_p)) {
     if (read(fd, buf, RB_IO_WAIT_READABLE_BUF) != -1) {
       return Qnil;
@@ -172,6 +173,7 @@ VALUE io_spec_rb_io_wait_readable(VALUE self, VALUE io, VALUE read_p) {
   }
 
   return ret ? Qtrue : Qfalse;
+# endif
 }
 #endif