Question: Warning, solutions may have been lost !!!

Hi, I got a problem that "Warning, solutions may have been lost" when I solve 

eqns := {a+r*t*ln(l)+(1-l)^2*m-b-r*t*ln(s)-(1-s)^2*n = 0, a+r*t*ln(1-l)+l^2*m-b-r*t*ln(1-s)-s^2*n = 0}
vars := {l, s}
solve(eqns, vars)
Why?
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