What might I be doing wrong to get "Error: s > 0 is not TRUE" for a nlmer model? <p>I am attempting to learn lme4, and I'll admit up front that my statistical academic training was in traditional ANOVA designs at the home of SAS (NC State) over 20 years ago. I now have about 4-5 years of experience with R, but mostly with traditional general linear models with all factors treated as fixed effects. </p> <p>I now would like to estimate the parameters for a von Bertalanffy growth model for an experiment with fresh-water mussels. Shell length was measured once per year for 4 years on known individuals (ID), so this is a repeated measures design. Ultimately I would like to add a main treatment factor and a split-plot factor. But, at the moment I can't get the simplified model to run. The R code that I'm using is:</p> <pre><code>VBGF &lt;- function(Linf, k, age, age.0) { Linf * (1-exp(-k*((age-age.0)))) } VBGF &lt;- deriv(L ~ VBGF, namevec=c("Lin", "k", "age.0"), function.arg=VBGF) mod1 &lt;- nlmer(L ~ VBGF(Linf,k,AGE,age.0) ~ (Linf+k+age.0|ID), mussels, start=c(50,0.1,-11), verbose=TRUE) </code></pre> <p>I get the following error, and I have been completely unsuccessful in finding a reference to what it means.</p> <pre><code>Error: s &gt; 0 is not TRUE </code></pre> <p>It's probably something simple that I'm overlooking, so I apologize for my ignorance in advance.</p>