can't read property inside .subscribe

I've a problem with a loop which actually works but give me this error in the console:

ERROR TypeError: Cannot read property 'X' of undefined

Here is the code:

for (let counter = 0; counter < this.signs.length; counter++) {

      if (this.sign === this.pairs[counter].value) {
        this.collectService.getSign(this.sign).subscribe(data => {
          // value X, I try and I export but give me an error
          this.X = data.sign.toString();
          this.changeComponent(this.sign1, this.sign2, this.X);


So, one more time, it works but why do I have this error?!