Thread: Assertion failure taking place of a try-catch

I know I can't create this iterator. What is not clear to me is why the try-catch block is failing to isolate the error and instead I get an assertion failure at runtime.

(note: vec1 has been properly declared and initiliazed)

Code:

/* ... */
#include <stdexcept>
/* ... */
try {
    vector<int>::const_iterator ivec = vec1.begin() - 1;
} catch (runtime_error err) {
    cout << err.what();
}