cannot be waived); 2 Moore’s, supra, § 8.07[3] (noting that affirmative defenses are, waivable); see also Denver & Rio Grande W.R. Co. v. Blackett, 538 F.2d 291, 294 (10th Cir.1976) (“[T]he application of affirmative defenses offer[s] no jurisdictional question.”)- While it is true, as Sanborn points out, that Colorado courts have barred civil suits against employers when workers’ compensation was available, in none of these cases did the parties dispute the existence of the state law bar, as they have done in the current case. See, e.g., Kandt v. Evans, 645 P.2d 1300, 1305-06 (Colo.1982) (affirming grant of summary judgment because plaintiff had already received workers’ compensation benefits and the fact that plaintiff was ac gomery LC, Inc., 942 P.2d 1230, 1234 (Colo.Ct.App.1996) (<HOLDING>); cf. Massie v. Godfather’s Pizza, Inc., 844
holding that the issue of foreseeability should be submitted to the jury where reasonable persons might differ holding that new york common law must be applied to determine whether the parties formed an oral settlement agreement and state statutory law must be applied to determine whether the agreement if any is enforceable holding that factual issues essential to determining whether state law bar applied should be submitted to a jury holding that the disputed issue with respect to the state law bar was properly submitted to the jury holding that state court with jurisdiction over  301 claim should have applied federal labor law rather than state contract law