title: Pulp.pulpTestAll() test failed on "Testing zero constraint"
body: <p>I am using anaconda version 1.6.14, python version 3.6.5, pulp version 1.6.8 and CPLEX version 12.8.</p>
  
  <p>When I run pulp.pulpTestAll() coin passes and CPLEX_CMD fails on testing zero constraint (see output below).</p>
  
  <p>The same output is given in this question: <a href="https://stackoverflow.com/questions/40838567/pulp-pulptestall-test-failed-too-many-values-to-unpack/51177351?noredirect=1#comment89337413_51177351">Pulp.pulpTestAll() test failed, too many values to unpack</a>
  But the question is asking why GUROBI failed not why CPLEX failed, and therefore the answers provided there do not answer my question.</p>
  
  <p>The output of pulp.pulpTestAll() is:</p>
  
  <pre><code>pulp.pulpTestAll()
           Testing zero subtraction
           Testing inconsistant lp solution
           Testing continuous LP solution
           Testing maximize continuous LP solution
           Testing unbounded continuous LP solution
           Testing Long Names
           Testing repeated Names
           Testing zero constraint
           Testing zero objective
           Testing LpVariable (not LpAffineExpression) objective
           Testing Long lines in LP
           Testing LpAffineExpression divide
           Testing MIP solution
           Testing MIP solution with floats in objective
           Testing MIP relaxation
           Testing feasibility problem (no objective)
           Testing an infeasible problem
           Testing an integer infeasible problem
           Testing column based modelling
           Testing dual variables and slacks reporting
           Testing fractional constraints
           Testing elastic constraints (no change)
           Testing elastic constraints (freebound)
           Testing elastic constraints (penalty unchanged)
           Testing elastic constraints (penalty unbounded)
  * Solver &lt;class 'pulp.solvers.PULP_CBC_CMD'&gt; passed.
  Solver &lt;class 'pulp.solvers.CPLEX_DLL'&gt; unavailable
           Testing zero subtraction
           Testing inconsistant lp solution
           Testing continuous LP solution
           Testing maximize continuous LP solution
           Testing unbounded continuous LP solution
           Testing Long Names
           Testing repeated Names
           Testing zero constraint
  '_dummy'
  * Solver &lt;class 'pulp.solvers.CPLEX_CMD'&gt; failed.
           Testing zero subtraction
           Testing inconsistant lp solution
           Testing continuous LP solution
           Testing maximize continuous LP solution
           Testing unbounded continuous LP solution
  Failure: status == -2 not in [-1]
  Failure: status == Unbounded not in ['Infeasible']
  Tests failed for solver &lt;pulp.solvers.CPLEX_PY object at 0x7f7aaef0f898&gt;
  * Solver &lt;class 'pulp.solvers.CPLEX_PY'&gt; failed.
  Solver &lt;class 'pulp.solvers.COIN_CMD'&gt; unavailable
  Solver &lt;class 'pulp.solvers.COINMP_DLL'&gt; unavailable
  Solver &lt;class 'pulp.solvers.GLPK_CMD'&gt; unavailable
  Solver &lt;class 'pulp.solvers.XPRESS'&gt; unavailable
  Solver &lt;class 'pulp.solvers.GUROBI'&gt; unavailable
  Solver &lt;class 'pulp.solvers.GUROBI_CMD'&gt; unavailable
  Solver &lt;class 'pulp.solvers.PYGLPK'&gt; unavailable
  Solver &lt;class 'pulp.solvers.YAPOSIB'&gt; unavailable
  </code></pre>