diff --git a/spec/core/exception/errno_spec.rb b/spec/core/exception/errno_spec.rb
index e11e2120ae..1ab4277700 100644
--- a/spec/core/exception/errno_spec.rb
+++ b/spec/core/exception/errno_spec.rb
@@ -38,9 +38,7 @@
   # From http://jira.codehaus.org/browse/JRUBY-4747
   it "is the same class as Errno::EWOULDBLOCK if they represent the same errno value" do
     if Errno::EAGAIN::Errno == Errno::EWOULDBLOCK::Errno
-      NATFIXME 'it is the same class as Errno::EWOULDBLOCK if they represent the same errno value', exception: SpecFailedException do
-        Errno::EAGAIN.should == Errno::EWOULDBLOCK
-      end
+      Errno::EAGAIN.should == Errno::EWOULDBLOCK
     else
       Errno::EAGAIN.should_not == Errno::EWOULDBLOCK
     end
@@ -54,9 +52,7 @@
 
   it "is the same class as Errno::EOPNOTSUPP if they represent the same errno value" do
     if Errno::ENOTSUP::Errno == Errno::EOPNOTSUPP::Errno
-      NATFIXME 'it is the same class as Errno::EOPNOTSUPP if they represent the same errno value', exception: SpecFailedException do
-        Errno::ENOTSUP.should == Errno::EOPNOTSUPP
-      end
+      Errno::ENOTSUP.should == Errno::EOPNOTSUPP
     else
       Errno::ENOTSUP.should_not == Errno::EOPNOTSUPP
     end
diff --git a/src/errno.rb b/src/errno.rb
index 3d9d481cd4..5dc72dc269 100644
--- a/src/errno.rb
+++ b/src/errno.rb
@@ -420,7 +420,13 @@ module Errno
 end
 
 SystemCallError::ERRORS.each do |name, (number, _desc)|
-  klass = Class.new(SystemCallError)
-  klass.const_set(:Errno, number)
-  Errno.const_set(name, klass)
+  first_name, = SystemCallError::ERRORS.find { |_name, (number2, _desc)| number == number2 }
+  if first_name != name
+    klass = Errno.const_get(first_name)
+    Errno.const_set(name, klass)
+  else
+    klass = Class.new(SystemCallError)
+    klass.const_set(:Errno, number)
+    Errno.const_set(name, klass)
+  end
 end
diff --git a/src/io.rb b/src/io.rb
index b082ffc565..97df698e05 100644
--- a/src/io.rb
+++ b/src/io.rb
@@ -153,12 +153,17 @@ class EAGAINWaitWritable < Errno::EAGAIN
     include IO::WaitWritable
   end
 
-  class EWOULDBLOCKWaitReadable < Errno::EWOULDBLOCK
-    include IO::WaitReadable
-  end
+  if Errno::EAGAIN.equal?(Errno::EWOULDBLOCK)
+    EWOULDBLOCKWaitReadable = EAGAINWaitReadable
+    EWOULDBLOCKWaitWritable = EAGAINWaitWritable
+  else
+    class EWOULDBLOCKWaitReadable < Errno::EWOULDBLOCK
+      include IO::WaitReadable
+    end
 
-  class EWOULDBLOCKWaitWritable < Errno::EWOULDBLOCK
-    include IO::WaitWritable
+    class EWOULDBLOCKWaitWritable < Errno::EWOULDBLOCK
+      include IO::WaitWritable
+    end
   end
 
   class EINPROGRESSWaitReadable < Errno::EINPROGRESS