[R] Error in nls Error in if(any(start < low || start > upp)) {: missing value when TRUE/FALSE needed

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Hi,

I got this error every time when I put upper and lower bound on my
parameters in nls. I'm not quite sure what this error is about. I also
wanted to find the content of nls_port_fit. But typing this directly
into R doesn't seem to give any result out.

I also tried to see the content of  If anybody could shed some light
I'd greatly appreciate. Thank you.

rc