Returning multiple solutions with CPLEX, 'bad suffix .npool'

I've tried generating multiple solutions with cplex using

option solver cplexamp;
option cplex_options 'poolstub=solfile populate=1 poolintensity=4';

...

for {k in K_mach_RESOURCES} {
    solve SUB1[k];
    for {l in 1..SUB1[k].npool}{
        solution ("solfile" & l & ".sol");
        display _varname, _var;
    }

Gives the error

    Bad suffix .npool for SUB1
context:  for {l in  >>> 1..SUB1[k].npool} <<< {
Possible suffix values for SUB1.suffix:
    astatus   exitcode   message   relax
    result    sstatus    stage  

The weird thing is that it's generating .sol files, but I don't know how to access the generated solutions! Possibly relevant info: there's multiple problems declared in the run file. Accessing Current.npool doesn't work either (in fact, it assumes Current is the latest DECLARED problem, not the latest SOLVED problem). Any ideas??

It seems as if the problem arose because the problem wasn't defined to be an INTEGER problem, but a LP-relaxation of an integer problem.

For some reason, CPLEX doesn't seem to support the populate method for linear programs.