Optimization throws exception

[Georg123]

uniform sampler2D _diffuseMap;
varying vec2 texcoord0;

void main()
{
vec4 diffuseColor = texture2D(_diffuseMap, texcoord0);
if (diffuseColor.a < 0.1)
discard;

gl_FragColor = vec4(diffuseColor.rgb, diffuseColor.a);

}

compileShader(glslSource, false, true, false) is throwing

[Fault] exception, information=Error: Expected token not seen was: ft1

Without optimization it works. The unoptimized AGAL code is:

tex ft0.xyzw, v0.xyyy, fs0 <2d,wrap,linear>
slt ft1.x, ft0.wwww, fc0.xxxx
neg ft1.x ft1.x
kil ft1.x

mov ft1.xyz, ft0.xyzz
mov ft1.w, ft0.wwww
mov oc.xyzw, ft1.xyzw

Regards,
Georg