Question: Eigenvalues of Symmetrix matrix

i am new user in maple .i want to calculate eigenvalues of a symmetrix matrix to find max or min value but i usually face a error code ' (in simpl/max) complex argument to max/min'

i know that a symmetrix matrix have real eigenvalues.

for example:
restart:with(LinearAlgebra):with(GraphTheory):interface(rtablesize=infinity):
SeidelMatrix:=proc(G::Graph) local J,B,A,S;
J:=Matrix(1..NumberOfVertices(G),1..NumberOfVertices(G),shape=constant[1]):
B:=IdentityMatrix(NumberOfVertices(G)):
A:=AdjacencyMatrix(G):
S:=J-B-MatrixScalarMultiply(A,2):
convert(S,Matrix,shape=symmetric,datatype=float):
return S:
end proc:
E:=sort(eval(Eigenvalues(SeidelMatrix(CycleGraph(7)),output='list'))):
(min,max)(E);
Error, (in simpl/min) complex argument to max/min: (1/3)*(-28+(84*I)*3^(1/2))^(1/3)+(28/3)/(-28+(84*I)*3^(1/2))^(1/3)-1/3