Addition of PIC 9 variable with space not causing ABEND S0C7

Question

I expected the ADD instruction in the following COBOL program to trigger an ABEND S0C7. However the program terminated normally and the ouput was quite surprising.

The SYSIN in the JCL

//SYSIN   DD *
5
//* There is only the character 5 in the SYSIN (well, followed by a newline character of course)

The COBOL program

WORKING-STORAGE SECTION.
77 X                     PIC 9(10).

PROCEDURE DIVISION.
    ACCEPT X
*   This addition did not trigger an ABEND S0C7
    ADD 1                TO X
    DISPLAY 'X = ' X
    STOP RUN
    .

The output

X = 5000000001

I then ran the program in a debugger

The ACCEPT statement moved a '5' in the leftmost character of X and spaces in the others on the right. So to be more specific, X looked like this after the ACCEPT:

'9         '   <-- These are the 10 characters of X
 0987654321    <-- These are the bytes numbered for clarity

I thought maybe the display of X of my debugger had a bug and showed me space characters instead of zeros but no. I checked the memory at X and it really contained nine bytes with the value 0x40 which are indeed ' ' characters in EBCDIC.

I then checked the assembly generated by the compiler

Well, I forgot to copy the exact output from my machine at work but here is the assembly code in pseudo language.

* ADD 1                TO X
    PACK  XP, X         * Pack X to XP
    AP    XP, ONEP      * XP += 1
    UNPK  XP, X         * Unpack XP to X

• ONEP is the address where 1 is stored in a packed format.

So, what am I missing here?

Thanks for your help.

EDIT #1 - February 26th - I checked the exact assembly code

Monday morning. Got the assembly code from my machine at work...

*     ADD 1       TO X
PKA   672(R13),152(10,R9)      PACK ASCII  Y,X
AP    682(6,R13),1072(1,R13)   ADD PACKED  Y,1
UNPK  152(10,R9),682(6,R13)    UNPACK      X,Y
OI    161(,R9),X'F0'

... and the compile option ZONEDATA(NOPFD) was used.