C++ :: Allegro Shooter Game Error

I started making a shooter game where enemies appear randomly and you have to shoot them. Everything works fine except sometimes whenever a bullet hits an enemy an error comes up that says "Expression: vector subscript out of range". It doesn't happen every time there is a collision, which confuses me. Here is the collision of the bullets with enemies.

for (unsigned int i = 0; i < bullets.size(); i++) {
if (bullets[i].type == 1) {
for (unsigned int j = 0; j < enemies.size(); j++) {
float distance = sqrt(pow(bullets[i].x - enemies[j].x, 2) + pow(bullets[i].y - enemies[j].y, 2));
if (distance < bullets[i].radius + enemies[j].radius) {
bullets.erase(bullets.begin() + i);
enemies.erase(enemies.begin() + j);
} } }