For some cases of spherical functions, like BRDF or a dot product between a sample direction and a surface normal, projecting them to spherical harmonics coefficients gives values over 1 or below -1, e.g. when the dot product is always 1 for any point on the sphere (any direction).
Thus my question - are such values (larger than 1 and/or less than -1) valid values for spherical coefficients? Should the spherical function be prepared somehow before projection, by doing some kind of normalization? Am I simply forgetting something when implementing "the thing" in my program?