Unity object stuck where it shouldn't

Here's the thing. I am making a 3D top-down game about cars on the road. Right now I have different-colored cuboids representing cars. They are position-locked in Y and rotation-locked on all three axis (axises? Sorry, English isn't my first language). The cuboid in question is controlled with the following code:

using UnityEngine;
using System.Collections;

[System.Serializable]
public class Boundary
{
    public float xMin, xMax, zMin, zMax;
}

public class PlayerController : MonoBehaviour 

public float speed;

public Boundary boundary;

void FixedUpdate ()
{
    float moveHorizontal = Input.GetAxis("Horizontal");
    float moveVertical = Input.GetAxis("Vertical");

    Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
    GetComponent<Rigidbody>().velocity = movement * speed;

    GetComponent<Rigidbody>().position = new Vector3 
        (
            Mathf.Clamp (GetComponent<Rigidbody>().position.x, boundary.xMin, boundary.xMax),
            0.0f,
            Mathf.Clamp (GetComponent<Rigidbody>().position.z, boundary.zMin, boundary.zMax)
        );
    }

}

It's pretty simple, almost unchanged code from the Unity tutorial. Boundaries are set as -8, 8 on X and -13, 13 on Z. The cuboid spawns at 0, 0.55 (It's 1 height, I gave it some clearance), -13. I move it forward with the WASD and it just gets stuck at 0.15 Z. Just dies there and can't move anywhere.

I'm very new to Unity, so I expect it to be some rookie mistake. Can someone help me out? Thank you.