Encountering an error when compiling and cannot understand what is wrong.

I have a short program just for testing out a servo I have. I am using an external power source and have the control wire hooked up to the ninth digital output pin.

Here is the code:

#include <Servo.h>

Servo myservo;

void setup() 
{
  myservo.attach(9);
}

void loop() 
{
  myservo.writeMicroseconds(1500);
  delay(1000);
  
  myservo.writeMicroseconds(1000);
  delay(1000);
  
  myservo.writeMicroseconds(2000);
  delay(1000);
}

And here is the error message:

Arduino: 1.6.4 (Windows 8.1), Board: “Arduino Uno”

C:\Users\Alex\AppData\Local\Temp\build2781569589806979821.tmp/core.a(wiring.c.o): In function delay': C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/wiring.c:114: undefined reference to yield’
collect2.exe: error: ld returned 1 exit status
Error compiling.

Any ideas as to what is causing it?

1.6.4 seems to have changed several things, possibly to tighten up previous sloppy use of C/C++ features and/or Arduino specific features, but it has also caused some problems where older programs and libraries used things that are no longer supported. This may be one of those cases. The code compiles OK under 1.5.6-r2