Question: Why does 'UseAssumptions' not account for 'real' property on parameters?

I tried the following:

Y9 := RandomVariable(LogNormal(a, b)):
assume(a::real, 0 < b):
c9:=solve({mu = Mean(Y9), sigma^2 = Variance(Y9)},{a,b}, UseAssumptions);


And received the following warning:

Warning, solve may not respect assumed property 'real' on 'a'.


How do I force Maple to account for property 'real' on 'a'?

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