Thread: unable to free string

When trying to execute this piece of code, the program craches when it reaches free(zin). When I remove the free(zin), everything works well, but I think it's better to free the allocated memory. Does anyone know what I'm doing wrong ?

Code:

void readFile(){
	FILE *f;
	char *zin, *tag;

	if ( !(f = fopen("test.txt", "r")) ) return;

	zin = malloc(sizeof(char));
	while (fgets(zin, SKY_MAX_ZIN_SIZE, f) != NULL){
		tag = fileZoekTag (zin);
		fileProcessTag(tag);
	}
	free (zin);
	fclose(f);
}

That was the problem indeed. Thanks.