Arduino Error, Char Array showing 11 chars not 12

I am trying to understand char arrays in the arduino, but my code is showing some strange results. Can anyone help:

typedef struct {char TAG[12];} stTAGdef12;
typedef struct {char TAG[13];} stTAGdef13;

stTAGdef12 st12a;
stTAGdef12 st12b;
stTAGdef13 st13a;
stTAGdef13 st13b;

void setup()
{
    //       01234567890123
    Add_s12("012345678901");
    Add_s13("0123456789012");
    Add_a12("012345678901");
    Add_a13("0123456789012");
    Serial.begin(9600);
}

void loop()
{
    Serial.print("\n12a >");Serial.print(st12a.TAG);Serial.print("<");
    Serial.print("\n13a >");Serial.print(st13a.TAG);Serial.print("<");
    Serial.print("\n12b >");Serial.print(st12b.TAG);Serial.print("<");
    Serial.print("\n13b >");Serial.print(st13b.TAG);Serial.print("<");
    Serial.print("\n12a if ");if (strcmp(st12a.TAG,"012345678901")   ==  0) Serial.print("Worked!!"); else Serial.print ("Failed");
    Serial.print("\n13a if ");if (strcmp(st13a.TAG,"0123456789012")  == 0) Serial.print ("Worked!!"); else Serial.print ("Failed");
    Serial.print("\n12b if ");if (strcmp(st12b.TAG, "012345678901")  == 0) Serial.print ("Worked!!"); else Serial.print ("Failed");
    Serial.print("\n13b if ");if (strcmp(st13b.TAG, "0123456789012") == 0) Serial.print ("Worked!!"); else Serial.print ("Failed");
    while(1){}
    /* Results
    connecting at 9600

    12a >01234567890<  (0-10)
    13a >012345678901< (0-11)
    12b >012345678901012345678901< ??
    13b >0123456789012< (0-12)
    12a if Failed
    13a if Failed
    12b if Failed
    13b if Worked!!
    */
}
void Add_s12(String input){
    input.toCharArray(st12a.TAG,12);
}
void Add_s13(String input){
    input.toCharArray(st13a.TAG,13);
}
void Add_a12(String input){
    for(int i=0;i<12;st12b.TAG[i]=input[i++]){}
}
void Add_a13(String input){
    for(int i=0;i<13;st13b.TAG[i]=input[i++]){}
}

Eventually I intend to save the data structures to the EEPROM as therefore cannot use strings. I do not understand what is going on with 12b, have I missed something?

Thanks in advance.

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