我尝试使用spei分析水平衡“ DataWaterBalance”的时间序列（从2013年1月到2017年12月，为期四个月的汇总）。我的结果中的所有NA都来自哪里（尤其是5月，6月和10月）？
这是我的数据集：
> DataWaterBalance
Jan Feb Mar Apr May Jun
2013 44.379560 39.407684 21.562829 -24.299970 47.503552 -37.464860
2014 53.067473 43.866450 -36.911638 -50.963592 -13.763198 -67.691191
2015 92.641825 35.884504 8.830966 -44.549551 -63.924239 -74.143715
2016 96.391935 82.837027 35.548819 12.616174 -19.708125 31.203054
2017 24.116392 50.623918 10.143826 -58.697190 -66.739512 -92.584874
Jul Aug Sep Oct Nov Dec
2013 -91.634212 -71.434604 31.166592 80.287287 83.802351 54.771282
2014 43.378232 68.080230 -40.233815 42.079559 41.352519 83.517715
2015 -80.702792 -3.758470 36.226553 3.940226 86.975420 38.772885
2016 -57.579186 -69.207669 -39.830591 22.390467 54.360755 9.304138
2017 6.082707 -20.811211 40.940572 35.029547 80.916921 108.675360
这些是我的结果：
> SPEI <- spei(DataWaterBalance ,scale =4)
> SPEI
Jan Feb Mar Apr May Jun Jul
2013 NA NA NA 0.33216038 NA NA 0.24659491
2014 NA NA NA -1.01494215 NA NA 0.44540983
2015 NA NA NA 0.47583043 NA NA -1.21875803
2016 NA NA NA 1.46151624 NA NA 1.12388023
2017 NA NA NA -0.59834429 NA NA -0.82578210
Aug Sep Oct Nov Dec
2013 0.08609257 -1.10921341 NA -0.12138666 0.81098577
2014 1.45984189 1.35213116 NA -0.64499892 -0.61207794
2015 -1.19831116 -0.10885634 NA -0.14754493 -0.21176235
2016 0.52253486 -0.35520876 NA -1.87008029 -1.26423284
2017 -0.22246731 0.71017826 NA 1.79802481 1.01404395
我做错了什么？ 谢谢！