How to check if a input is a string and show validation

I'm making a program in which the user inputs numbers into an entry box. I want to check its a number and if a letter has been entered show an error message however the program crashes instead of shows the error message. This is my code, any help?

def BreakEven(): #makes the breakeven chart 
print("break even code goes here")

global e1text
if int(e1text) >= 101 or e1text.isdigit()== False:
    tkinter.messagebox.showerror('Break Even','Total Revenue Invalid input')#makes error messages when no number is entered or more than 100
    command = closewindow()
print(e1text)