How do you generate an RGBA image in Core Graphics?

I'm trying to generate an RGBA8 image from text to use as an OpenGL ES 2.0 texture.

+(UIImage *)imageFromText:(NSString *)text
{
  UIFont *font = [UIFont systemFontOfSize:20.0];  
  CGSize size  = [text sizeWithFont:font];

  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
  CGContextRef contextRef =  CGBitmapContextCreate (NULL,
                                                    size.width, size.height,
                                                    8, 4*size.width,
                                                    colorSpace,
                                                    kCGImageAlphaLast
                                                    );
  CGColorSpaceRelease(colorSpace);
  UIGraphicsPushContext(contextRef);

  [text drawAtPoint:CGPointMake(0.0, 0.0) withFont:font];
  UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

  UIGraphicsPopContext();

  return image;
}

Unfortunately, there's no CGColorSpaceCreateDeviceRGBA, and CGColorSpaceCreateDeviceRGB results in the following error:

CGBitmapContextCreate: unsupported parameter combination: 8 integer bits/component; 32 bits/pixel; 3-component color space; kCGImageAlphaLast; 448 bytes/row.

What am I missing to create the proper RGBA8 format that OpenGL wants here?

Update: I changed the last parameter of CGBitmapContextCreate from kCGImageAlphaNone (which it was when I copy pasted the code) to kCGImageAlphaLast, which is one of several variations I've tried in error.

Update 2: UIGraphicsGetImageFromCurrentImageContext() returns nil if the context was not created with UIGraphicsBeginImageContext(), so it is necessary to extract the image differently: [UIImage imageWithCGImage:CGBitmapContextCreateImage(contextRef)].