Error when trying to use NTL vector

Question

I am trying to use NTL::Vec, but I get this error whenever I want to use SetLength()

The c++ code:

#include <NTL/ZZ.h>
#include <NTL/vector.h>

int main(int argc, const char * argv[]) {

NTL::Vec<NTL::ZZ> v;
v.SetLength(8);
return 0;
}

The error message:

/usr/include/NTL/ZZ.h:78: undefined reference to `_ntl_gfree(void**)'
./main.o: In function `NTL::Vec<NTL::ZZ>::AllocateTo(long)':
/usr/include/NTL/vector.h:334: undefined reference to `NTL::Error(char   const*)'
/usr/include/NTL/vector.h:337: undefined reference to `NTL::Error(char const*)'
/usr/include/NTL/vector.h:343: undefined reference to `NTL::Error(char const*)'
/usr/include/NTL/vector.h:354: undefined reference to `NTL::Error(char const*)'
/usr/include/NTL/vector.h:369: undefined reference to `NTL::Error(char const*)'
collect2: error: ld returned 1 exit status

If I don't use v.SetLength(8), I don't get an error and everything is fine. What could be the problem ?

Thank you in advance