Question: improper op or subscript selector calling solve.

I am getting this error. Is this expected or known issue?  

restart;
sol:=-csgn(1, 1/(_C1*a - _C1*x - 1))*_C1*a^4/((k + 1)*(_C1*a - _C1*x - 1)^2) + 4*csgn(1, 1/(_C1*a - _C1*x - 1))*_C1*a^3*x/((k + 1)*(_C1*a - _C1*x - 1)^2) - 6*csgn(1, 1/(_C1*a - _C1*x - 1))*_C1*a^2*x^2/((k + 1)*(_C1*a - _C1*x - 1)^2) + 4*csgn(1, 1/(_C1*a - _C1*x - 1))*_C1*a*x^3/((k + 1)*(_C1*a - _C1*x - 1)^2) - csgn(1, 1/(_C1*a - _C1*x - 1))*_C1*x^4/((k + 1)*(_C1*a - _C1*x - 1)^2) + a^2/((k + 1)*(_C1*a - _C1*x - 1)^2) - 2*a*x/((k + 1)*(_C1*a - _C1*x - 1)^2) + x^2/((k + 1)*(_C1*a - _C1*x - 1)^2) + csgn(1, 1/(_C1*a - _C1*x - 1))*a^3/((k + 1)*(_C1*a - _C1*x - 1)^2) - 3*csgn(1, 1/(_C1*a - _C1*x - 1))*a^2*x/((k + 1)*(_C1*a - _C1*x - 1)^2) + 3*csgn(1, 1/(_C1*a - _C1*x - 1))*a*x^2/((k + 1)*(_C1*a - _C1*x - 1)^2) - csgn(1, 1/(_C1*a - _C1*x - 1))*x^3/((k + 1)*(_C1*a - _C1*x - 1)^2) - csgn(1/(_C1*a - _C1*x - 1))*a^2/((k + 1)*(_C1*a - _C1*x - 1)^2) + 2*csgn(1/(_C1*a - _C1*x - 1))*a*x/((k + 1)*(_C1*a - _C1*x - 1)^2) - csgn(1/(_C1*a - _C1*x - 1))*x^2/((k + 1)*(_C1*a - _C1*x - 1)^2);

solve( simplify(sol)=0,x,allsolutions = true) assuming real; #also x::real, same error

Maple 2020.2 on winsows 10. Physics 897