undefined symbol for LoopPass

I am building a loop pass following these instructions: http://llvm.org/docs/WritingAnLLVMPass.html
Everything works fine, I did it many times for Function Passes, but in the runOnLoopmethod, whenever I call a method of the loop L passed as argument, for example L->begin(), I get the following error:

opt: symbol lookup error: /home/giacomo/llvmcsfv/Debug+Asserts/lib/Acsl.so: undefined symbol: _ZNK4llvm8LoopBaseINS_10BasicBlockENS_4LoopEE5beginEv

Where Acsl is the name of the loadable module. If I remove all the instructions from runOnPass but a debug print, it works fine (it prints it), so the problem is not the module.
Does anybody have any Idea?
Thank you very much,