Quadratic programming maximization -- matrix not positive definite

Question

I am trying to optimize the following simple objective function using the quadprog program in R:

max_{x} x'Ax

Most optimization problems I see use minimization, but if I simply use -A instead of A, I get an error that A is no longer positive definite. I am very new to this kind of thing. Does anyone know how to solve a simple quadratic maximization problem?

Here is sample code to reproduce the error:

if (!require(quadprog)) install.packages('quadprog')
library(quadprog)
set.seed(144)
mat <- abs(cor(matrix(rnorm(25),5,5)))
solve.QP(Dmat = -mat, dvec = rep(0,5), Amat = diag(5), bvec = rep(0,5), meq=0, factorized=FALSE)

Here is the resulting error:

Error in solve.QP(Dmat = -mat, dvec = rep(0, 5), Amat = diag(5), bvec = rep(0, : matrix D in quadratic function is not positive definite!