Why `top` shows incorrect(showm as question mark ) `VIRT` memory for larger 8TB size ?

Issue

• The top command shows ? if process is using more that 8 TB virtual memory,

  #: top -b -n1 | grep a.out
  PID   USER PR  NI  VIRT  RES   SHR S %CPU %MEM    TIME+    COMMAND
  47395 root 20   0  8150g 1104  380 S  0.0  0.0    0:00.00  a.out    
  

  • Next allocation(> 8 TB)

  #: top -b -n1 | grep a.out
  PID   USER PR  NI  VIRT  RES   SHR S %CPU %MEM   TIME+    COMMAND
  47395 root 20   0     ?  1112  380 S  0.0  0.0   0:00.00  a.out      
  

  • Here a.out is a C program which allocates virtual memory(50 GB at time).