Keypad comes up when Galaxy S5 receives a phone call but won't go away so can't answer call. How can I fix this?

[AC Question]

Keypad comes up when Galaxy S5 recieves a phone call but won't go away so can't answer call

It doesn't happen every time, but sometimes the keypad comes up when my Galaxy S5 receives a call and even if I put my password in it doesn't go away so I can't answer the call. Any ideas?