weird "empty" character

[DyslexicChciken]

My terminal prints the words read from a document and stores it in binary search tree object called document1, and their corresponding lines numbers in the document are also printed.
____________________________________________

document1.InorderTraverseofBinarySearchTree(display) returns:
@@: 6, 52, 76, 143, 144
@@a: 10, 12, 14, 15, 15, 20, 21, 22, 24, 26, 30, 33, 34, 35, 36, 37, 40, 45, 50, 51, 55, 58, 76, 79, 86, 86, 88, 89, 95, 95, 114, 117, 130, 134, 149, 151, 151, 163, 172, 173, 174, 175, 179, 182, 187, 188
@@able: 30
@@about: 190
@@accessnumbers: 148
@@according: 75
@@acknowledged: 68

//...
____________________________________________

As you can see it prints this weird "word" that occurs on lines 6, 52, etc:

@@: 6, 52, 76, 143, 144

The word doesn't exist. I made sure I only use alphabets by using isalpha(input_string[i]) from cctype.h, but somehow I still get this weird empty character. I already checked for '\0' using:

else if(input_string[i] == ' ' && input_string != " " && input_string[0] != '\0')

I also went through the document and found no such "empty character". What character could this be?

[koothkeeper]

I'd like to see your code. BTW, I dig your handle!