Error message in ETA vs. COV plot in xpose4

[Pradeep2986]

Hi,
I am using Xpose4 function for Etas_vs_Covariates to generate the the plot. Most of the ETA plots look fine, but for one of them I get the plot below. The error message seems to indicate I have non numeric value in the data for ETA 5 . But other ETA plots vs DOSA look fine. I thought may be this is due to the variability/sparsity of the data to allow fitting smooth line, so tried to turn off the argument as follows:
ranpar.vs.cov(xpdb, smooth = FALSE)

But, the error still appears and also all other ETA plots still have the smooth line.

What is the cause of this error?
Why smooth cannot be turned off with the argument above?
Is there a workaround for this?

Thanks for any comments.
Pradeep