Shadowmaps and Cubemaps incompatible ?

I want to do shadowmapping with cubemaps
wen i want to set the parameters:
GL_TEXTURE_COMPARE_SGIX, GL_TRUE
GL_TEXTURE_COMPARE_OPERATOR_SGIX, GL_TEXTURE_LEQUAL_R_SGIX

i get a GL_INVALID_VALUE error.

Are cubemaps incompatible with this format ?