How to count only letters in string?

I am trying to write a program that counts several elements in a string. The first one of those being letters.

The assignment is part of the CS50 week 2 problem set, hence the libraries included.

Using the while condition I was able to count every single character, but the code stopped working once I´ve added isalnum (which checks if the character is alphanumerical).

What am I doing wrong?

#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>

int main(void) {
    string text = get_string("Text: ");
    printf("%s, \n", text);

    int letters = 0;
    while (text[letters] != '\0') {
        if (isalnum(text[letters])) {
            letters++;
        }
    }
    printf("%i \n", letters);
}

Here is shown how a correct loop can be defined

size_t letters = 0;
for ( size_t i = 0; text[i] !='\0'; i++ )
{
    if ( isalnum( ( unsigned char )text[i] ) )
    {
       letters++;
    }
}

printf( "%zu\n", letters );

If you want to use a while loop then it can look for example like

size_t letters = 0;
size_t i = 0;
while ( text[i] !='\0' )
{
    if ( isalnum( ( unsigned char )text[i++] ) )
    {
       letters++;
    }
}

printf( "%zu\n", letters );

Pay attention to that the function isalnum detects letters and digits. If you need to count only letters then use the function isalpha.