Error: function cannot be evaluated at initial parameters

I have an error for a simple optimization problem. Is there anyone knowing
about this error?

lambda1=-9
lambda2=-6
L<-function(a){
s2i2f<-(exp(-lambda1*(250^a)-lambda2*(275^a-250^a))
       -exp(-lambda1*(250^a)-lambda2*(300^a-250^a)))
logl<-log(s2i2f)
return(-logl)}
optim(1,L)

Error in optim(1, L) : function cannot be evaluated at initial parameters

Thank you in advance

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