stphtest -schoenfeld residuals plotted for first 2 variables, for 3rd variable 'variable not found'

[Martin Imelda Borg]

Hi I was assessing Ph for my finalised cox model . I created schoenfeld residuals

stcox var1 var2 i,var3 i.var4, schoenfeld(schoen*) scaledsch(scaled*)
stphtest, rank detail  //gives GOF
stphest, rank plot(var1)
stphtest, rank plot(var2) 
stphtest, rank plot (i.var3) ///variable not found error
stephtest, rank plot (i.var4) ///vraiable not found error

Why did it manage to plot var1 and var 2 without issues
However when I proceeded with categorical var 3 and var 4. It said variable not found ?
Is it coincidence that this happened for both the cateogircal variables but not for the conintous variables?