Is a failure to instruct on a lesser-included offense prejudicial where it strikes at the heart of the defense?

The failure to instruct on a lesser-included offense is more likely to be prejudicial where it strikes at the heart of the defense. ( People v. Neidinger (2006) 40 Cal.4th 67, 79, 51 Cal.Rptr.3d 45, 146 P.3d 502 ; see People v. McDonald (1984) 37 Cal.3d 351, 376, 208 Cal.Rptr. 236, 690 P.2d 709 ["An error that impairs the jurys determination of an issue that is both critical and closely balanced will rarely be harmless."].)