GPUImage shader crashing with "ERROR: One or more attached shaders not successfully compiled"

I'm trying to build a Vibrance filter for GPUImage based on this Javascript:

/**
 * @filter       Vibrance
 * @description  Modifies the saturation of desaturated colors, leaving saturated colors unmodified.
 * @param amount -1 to 1 (-1 is minimum vibrance, 0 is no change, and 1 is maximum vibrance)
 */
function vibrance(amount) {
    gl.vibrance = gl.vibrance || new Shader(null, '\
        uniform sampler2D texture;\
        uniform float amount;\
        varying vec2 texCoord;\
        void main() {\
            vec4 color = texture2D(texture, texCoord);\
            float average = (color.r + color.g + color.b) / 3.0;\
            float mx = max(color.r, max(color.g, color.b));\
            float amt = (mx - average) * (-amount * 3.0);\
            color.rgb = mix(color.rgb, vec3(mx), amt);\
            gl_FragColor = color;\
        }\
    ');

    simpleShader.call(this, gl.vibrance, {
        amount: clamp(-1, amount, 1)
    });

    return this;
}

One would think I should be able to more/less copy paste the shader block:

GPUImageVibranceFilter.h

@interface GPUImageVibranceFilter : GPUImageFilter
{
    GLint vibranceUniform;
}

// Modifies the saturation of desaturated colors, leaving saturated colors unmodified.
// Value -1 to 1 (-1 is minimum vibrance, 0 is no change, and 1 is maximum vibrance)
@property (readwrite, nonatomic) CGFloat vibrance;

@end

GPUImageVibranceFilter.m

#import "GPUImageVibranceFilter.h"

#if TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE
NSString *const kGPUImageVibranceFragmentShaderString = SHADER_STRING
(
    uniform sampler2D inputImageTexture;
    uniform float vibrance;
    varying highp vec2 textureCoordinate;
    void main() {
        vec4 color = texture2D(inputImageTexture, textureCoordinate);
        float average = (color.r + color.g + color.b) / 3.0;
        float mx = max(color.r, max(color.g, color.b));
        float amt = (mx - average) * (-vibrance * 3.0);
        color.rgb = mix(color.rgb, vec3(mx), amt);
        gl_FragColor = color;
    }
);
#else
NSString *const kGPUImageVibranceFragmentShaderString = SHADER_STRING
(
    uniform sampler2D inputImageTexture;
    uniform float vibrance;
    varying vec2 textureCoordinate;
    void main() {
        vec4 color = texture2D(texture, textureCoordinate);
        float average = (color.r + color.g + color.b) / 3.0;
        float mx = max(color.r, max(color.g, color.b));
        float amt = (mx - average) * (-vibrance * 3.0);
        color.rgb = mix(color.rgb, vec3(mx), amt);
        gl_FragColor = color;
    }
);
#endif

@implementation GPUImageVibranceFilter

@synthesize vibrance = _vibrance;

#pragma mark -
#pragma mark Initialization and teardown

- (id)init;
{
    if (!(self = [super initWithFragmentShaderFromString:kGPUImageVibranceFragmentShaderString]))
    {
        return nil;
    }

    vibranceUniform = [filterProgram uniformIndex:@"vibrance"];
    self.vibrance = 0.0;

    return self;
}

#pragma mark -
#pragma mark Accessors

- (void)setVibrance:(CGFloat)vibrance;
{
    _vibrance = vibrance;

    [self setFloat:_vibrance forUniform:vibranceUniform program:filterProgram];
}

@end

But that doesn't compile, crashing with:

Failed to compile fragment shader
Program link log: ERROR: One or more attached shaders not successfully compiled
Fragment shader compile log: (null)
Vertex shader compile log: (null)

The error is certainly clear, but being inexperienced with OpenGL ES shaders, I have no idea what the problem actually is.

via Chebli Mohamed