Intel 8051 UART mode 0 doesn't work in Proteus (clock impulses on TXD occur constantly, MCU recieves bytes itself)

I think that I’ve read everything about 8051′s UART but I can’t understand what goes wrong even in this simpliest example. According to literature in mode 0 MCU transmits byte only when we write it into SBUF, and only there 8 clock impulses occur in TXD line; MCU reads byte when TXD voltage changes.

In this example (only indicator is connected to P1) I write nothing to SBUF, but pediodically TI, then RI interrpupt occur, TXD voltage constantly changes and I constantly recieve 0.
Clock frequency is 12 MHz. What can be a reason?

Thank everyone for any effort!

;====================================================================
; MCU: 80C51
; Compiler:  ASEM-51 (Proteus)
;====================================================================

$NOMOD51
$INCLUDE (8051.MCU)

;====================================================================
; RESET AND INTERRUPT VECTORS
;====================================================================

      ; Reset vector
      org   0000h
      jmp   Init

      ; Serial port interrupt vector
      org   0023h
      call  SERIAL_PORT_HANDLER
      reti

;====================================================================
; MAIN CODE BLOCK
;====================================================================

      org   0150h
Init:   
      setb  ES
      setb  EA
      mov   SCON,       #00010000b ; UART Mode 0,
                       ; accept messages with any 9th bit,
                       ; enable recieving,
                       ; don't recieve 9th bit, 
                       ; clear interrupt flags

Loop:
      jmp   Loop

;====================================================================
; SERIAL PORT INTERRUPT HANDLER
;====================================================================      

SERIAL_PORT_HANDLER:
   jb       TI,     _SENT

_RECEIVED:
   mov      P1, SBUF ; Move to indicator
   clr      RI
   jmp      _AFTER_SERIAL_PORT_HANDLING

_SENT:
   clr      TI

_AFTER_SERIAL_PORT_HANDLING:
   ret

;====================================================================
      END