[R] loess with missing data points

[1Rnwb]

Hello List,
I have a longitudinal samples on multiple individuals, for initial analysis,
i am doing some plots to see how the variable changes with the age. In some
of the individuals one or two data points are missing, 
   IL1Ra              Age.at.Sample.Collection              Subject.ID
 6.575466           2.004106                                   00709-0
 NA                    2.162902                                   00709-0
5.115421            3.312799                                   00709-0
6.119626            3.515400                                   00709-0
5.705648            4.156057                                   00709-0
7.298887            4.405201                                   00709-0
6.195063            4.577686                                   00709-0
6.574864            5.541409                                   00709-0
When I run
cap<-paste(sub.id,cases2$group[1],sep=" ")
plot(cases2$IL1Ra~cases2$Age.at.Sample.Collection, ylab="eo_conc",xlab="Age
at Sample",pch=1, ylim=c(3,10), xlim=c(0,22),main=cap)
points(cases2$MIP1b~cases2$Age.at.Sample.Collection,pch=2,ylim=c(1.5,10),col="red")
points(cases2$MCP1~cases2$Age.at.Sample.Collection,,pch=3,
ylim=c(1.5,10),col="brown")
lines(predict(loess(cases2$IL1Ra~cases2$Age.at.Sample.Collection))~cases2$Age.at.Sample.Collection,
lwd=1, lty=1)
lines(predict(loess(cases2$MIP1b~cases2$Age.at.Sample.Collection))~cases2$Age.at.Sample.Collection,
col="red", lwd=1, lty=1)
lines(predict(loess(cases2$MCP1~cases2$Age.at.Sample.Collection))~cases2$Age.at.Sample.Collection,col="brown",
lwd=1, lty=1)

I get error
Error in model.frame.default(formula = predict(loess(cases2$IL1Ra ~
cases2$Age.at.Sample.Collection)) ~  : 
  variable lengths differ (found for 'cases2$Age.at.Sample.Collection')
In addition: There were 50 or more warnings (use warnings() to see the first
50)

I tried using na.action=na.omit, which is not working at all.

I would appreciate some pointers in this regards to get these charts.
Thanks

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