title: Gekko optimisation error with a nonlinear function and 2 inputs
body: <p>I want to maximise a concave function with two inputs</p>
  <p>max 2 * x1 ** .8 + 1.4 * x2 ** .9</p>
  <p>st x1 + x2 == C</p>
  <p>using Gekko, but I get a error code -2.</p>
  <pre><code> 
  from gekko import GEKKO
  
  m = GEKKO()
  
  C = m.Param(value=10)
  
  x1, x2 = [m.Var(lb=0, ub=10) for i in range(2)]
  
  x1.value = 5
  x2.value = 5
  
  m.Equation(x1 + x2 == C)
  
  m.Obj(2 * x1 ** .8 + 1.4 * x2 ** .9)
  
  m.options.IMODE = 3
  
  m.solve()
  
  print(x1.value)
  print(x2.value)
  </code></pre>