朴素贝叶斯函数fitcnb报错

错误使用 ClassificationNaiveBayes/fitNonMNDists (line 222)
A normal distribution cannot be fit for the combination of class Subject-1 and predictor x1. The data has zero variance.

出错 ClassificationNaiveBayes (line 104)
                this.DistributionParameters = fitNonMNDists(this);

出错 classreg.learning.FitTemplate/fit (line 258)
            [varargout{1:nargout}] = this.MakeFitObject(X,Y,W,this.ModelParams,fitArgs{:});

出错 ClassificationNaiveBayes.fit (line 132)
            this = fit(temp,X,Y);

出错 fitcnb (line 243)
    this = ClassificationNaiveBayes.fit(X,Y,RemainingArgs{:});

出错 final (line 67)
    tree = fitcnb(ff,label);

二十张图片，二十个标签，为什么会提示zero variance？我加了噪声，增加到了100张图片，还是一样的结果