Segmentation fault while using smart pointers

I get an error of -> Segmentation fault while accessing address 0x00007ff8bf611980 (_ZNSt10_weakptrIN10prometheus11CollectableELN9_gnu_cxx12_Lock_policyE2EEC1ERKS4_+0x1b) [0x7fff8bee28215]

My code lies below-

 void Family::MakeGauge3(std::string s1, std::string s2, const 
 std::map<string,string>& labels, MyExposer* ex)
 {  auto registry = make_shared<Registry>();
    gauge_family= &(BuildGauge().Name(s1).Help(s2).
    Labels(labels).Register(*registry));
    ex->register(registry); //problematic line
 }

The register function in the MyExposer class is->

 void Exposer::register(std::shared_ptr<Registry> reg)
 {
 exp ->RegisterCollectable(reg);
 }

exp is a private member of class MyExposer defined as->

Exposer* exp; //Exposer is another class

 void Exposer::RegisterCollectable(const std::weak_ptr<Collectable>& 
 collectable)
 {   collectables_.push_back(collectable);
 }

where collectable_is a private member defined in Exposer class as->

std::vector<std::weak_ptr<Collectable>> collectables_;