customtkinter ValueError: invalid literal for int() with base 10: ''

Question

I wrote a simple program where need get number and do some math problem such as the perimeter and area of ​​a triangle.

It shows an error:

Traceback (most recent call last):
    File "D:\\AAA\\Custom.py", line 154, in \<module\>
    command=Matem()
    File "D:\\AAA\\Custom.py", line 8, in Matem
    a=int(a1)
    ValueError: invalid literal for int() with base 10: ''

Her is my code, I think I wrote all correct but I do not understand why it does not work:

def Matem():
    a1 = (exaA_lbl.get())
    b1 = (exaB_lbl.get())
    c1 = (exaC_lbl.get())
    a=int(a1)
    b=int(b1)
    c=int(c1)
    P = a+b+c
    S = math.sqrt(P\*(P-a)*(P-b)*(P-c))
    exaP_lbl.insert(0,P)
    exaS_lbl.insert(0,S)
    if(a==b==c):
       exaINFO_lbl.insert(0,'a==b==c')
    elif(a!=b==c):
       exaINFO_lbl.insert(0, 'a!=b==c')
    elif(a==b!=c):
       exaINFO_lbl.insert(0, 'a==b!=c')
    elif(a!=b!=c):
       exaINFO_lbl.insert(0, 'a!=b!=c')
    else:
       exaINFO_lbl.insert(0, 'a!=c==b')