Two masses connected by spring with error: setting an array element with a sequence

I am working on a code simulating two masses connected by a spring in Python. However, somehow I always get the ValueError: setting an array element with a sequence. Can someone help me figuring out what the problem is here? I have gone through other answers on this forum but it did not help.

import numpy as np

k = 1.95
m1 = 0.1
m2 = 0.3
x10 = 0.03
x20 = 0.01
v10 = 0
v20 = 0
t0 = 0
tf = 25
dt = 0.05
x0 = 5
k1_drag = 0.2 

t = np.arange(t0, tf, dt)
x1 = np.zeros_like(t)
x2 = np.zeros_like(t)
v1 = np.zeros_like(t)
v2 = np.zeros_like(t)
a1 = np.zeros_like(t)
a2 = np.zeros_like(t)

def F1(x1, x2, x0 = 5):
    return k*(x2 - x1 -x0) - (k1_drag/m1) * v1

def F2(x1, x2, x0 = 5):
    return -k*(x2 - x1 - x0) - (k1_drag/m2) * v2


x1[0] = x10
x2[0] = x20
v1[0] = v10
v2[0] = v20
t[0] = t0

for i in range(1, len(t)):
    a1[i-1] = F1(x1[i-1], x2[i-1])
    a2[i-1] = F2(x1[i-1], x2[i-1])
    v1[i] = v1[i-1] + a1[i-1]*dt
    v2[i] = v2[i-1] + a2[i-1]*dt
    x1[i] = x1[i-1] + v1[i-1]*dt
    x2[i] = x2[i-1] + v2[i-1]*dt
    t[i] = t[i-1] + dt

F1 returns a sequence, which cannot be assigned to array element.