PVS-Studio warning V547 for MySQL

enum enum_mysql_timestamp_type
str_to_datetime(....)
{
  ...
  else if (str[0] != 'a' || str[0] != 'A')
    continue; /* Not AM/PM */
  ...
}

This suspicious code was found in MySQL project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'str [0] != 'a' || str [0] != 'A'' is always true. Probably the '&&' operator should be used here. clientlib my_time.c 340

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