diff --git a/spec/ruby/optional/capi/ext/io_spec.c b/spec/ruby/optional/capi/ext/io_spec.c
index 40069bd54b..1b47830fa4 100644
--- a/spec/ruby/optional/capi/ext/io_spec.c
+++ b/spec/ruby/optional/capi/ext/io_spec.c
@@ -218,6 +218,17 @@ VALUE io_spec_rb_io_close(VALUE self, VALUE io) {
 }
 #endif
 
+/*
+ * this is needed to ensure rb_io_wait_*able functions behave
+ * predictably because errno may be set to unexpected values
+ * otherwise.
+ */
+static VALUE io_spec_errno_set(VALUE self, VALUE val) {
+  int e = NUM2INT(val);
+  errno = e;
+  return val;
+}
+
 void Init_io_spec(void) {
   VALUE cls = rb_define_class("CApiIOSpecs", rb_cObject);
 
@@ -296,6 +307,8 @@ void Init_io_spec(void) {
 #ifdef HAVE_RB_CLOEXEC_OPEN
   rb_define_method(cls, "rb_cloexec_open", io_spec_rb_cloexec_open, 3);
 #endif
+
+  rb_define_method(cls, "errno=", io_spec_errno_set, 1);
 }
 
 #ifdef __cplusplus
diff --git a/spec/ruby/optional/capi/io_spec.rb b/spec/ruby/optional/capi/io_spec.rb
index 466bfe5a41..a832c6a93b 100644
--- a/spec/ruby/optional/capi/io_spec.rb
+++ b/spec/ruby/optional/capi/io_spec.rb
@@ -236,6 +236,7 @@ describe "C-API IO function" do
 
   describe "rb_io_wait_writable" do
     it "returns false if there is no error condition" do
+      @o.errno = 0
       @o.rb_io_wait_writable(@w_io).should be_false
     end
 
@@ -254,6 +255,7 @@ describe "C-API IO function" do
   platform_is_not :windows do
     describe "rb_io_wait_readable" do
       it "returns false if there is no error condition" do
+        @o.errno = 0
         @o.rb_io_wait_readable(@r_io, false).should be_false
       end
 
@@ -271,6 +273,7 @@ describe "C-API IO function" do
           @w_io.write "rb_io_wait_readable"
         end
 
+        @o.errno = Errno::EAGAIN.new.errno
         @o.rb_io_wait_readable(@r_io, true).should be_true
         @o.instance_variable_get(:@read_data).should == "rb_io_wait_re"