# Possible bug: GLPKError/assertion failed

Hello, Sage Community!

Recently, while trying to help one of the users of this forum, I wrote this code. As you can see, the MILP is defined as expected.

That was a few weeks ago. Just now, I have noticed that adding `p.solve()`

produces an error/exception:

GLPKError: Assertion failed: j->lb < j->ub

Error detected in file glpnpp03.c at line 2134

According to this, the problem seems to be in a C (maybe Cython) subroutine, which I am not able to modify. The strange thing is that defining this same MILP, manually, using the data showed there, does not produce any error! That what makes me think this is a bug.

Here is the complete traceback:

```
---------------------------------------------------------------------------
GLPKError Traceback (most recent call last)
<ipython-input-1-fbad4ef266dd> in <module>()
9 p.add_constraint(B[i], min=bmin[i], max=bmax[i])
10 p.show() # this is just to confirm we did it right
---> 11 p.solve()
/home/sc_serv/sage/local/lib/python2.7/site-packages/sage/numerical/mip.pyx in sage.numerical.mip.MixedIntegerLinearProgram.solve (build/cythonized/sage/numerical/mip.c:15815)()
2259 """
2260 if log is not None: self._backend.set_verbosity(log)
-> 2261 self._backend.solve()
2262 return self._backend.get_objective_value()
2263
/home/sc_serv/sage/local/lib/python2.7/site-packages/sage/numerical/backends/glpk_backend.pyx in sage.numerical.backends.glpk_backend.GLPKBackend.solve (build/cythonized/sage/numerical/backends/glpk_backend.c:9430)()
1060 if ((self.simplex_or_intopt == glp_intopt_only)
1061 or (self.simplex_or_intopt == glp_simplex_then_intopt) and (solution_status != GLP_UNDEF) and (solution_status != GLP_NOFEAS)):
-> 1062 sig_on()
1063 solve_status = glp_intopt(self.lp, self.iocp)
1064 solution_status = glp_mip_status(self.lp)
GLPKError: Assertion failed: j->lb < j->ub
Error detected in file glpnpp03.c at line 2134
```

Thanks in advance for your answers!

It's working well with

`cplex`

and we get the obvious solution`x[i] = 0`

. I tried with`PPL`

which is complaining that bounds must be constant, i.e., not`oo`

. So I used`10`

instead of`oo`

and solved it with`PPL`

. Then I tried again with`GLPK`

using`10`

instead of`oo`

and got a solution. So there might be something wrong in the way the backend handles`oo`

.