MSP430F5509 ADC Error reading

Other Parts Discussed in Thread: MSP430F5509

Hi I am a newbie in using MSP430F5509 and I have a problem in reading the ADC input value.

Here is my code. 

#include "msp430.h"

unsigned int ADC_Result;

int main()
{
  volatile unsigned int i;

  WDTCTL = WDTPW + WDTHOLD;                   // Stop WDT
  P6SEL |= BIT0;

  // Configure ADC
  ADC10CTL0 |= ADC10SHT_2 + ADC10ON;        // ADC10ON, S&H=16 ADC clks
  ADC10CTL1 |= ADC10SHP;                    // ADCCLK = MODOSC; sampling timer
  ADC10CTL2 |= ADC10RES;                    // 10-bit conversion results
  ADC10MCTL0 |= ADC10INCH_0;                // A0 ADC input select; Vref=AVCC
  ADC10IE |= ADC10IE0;                      // Enable ADC conv complete interrupt
  ADC10CTL0 |= ADC10ENC;


  P1DIR |= BIT0;                            // Set P1.0/LED to output direction
  P1OUT&= ~BIT0;

  while (1)
  {
    ADC10CTL0 |= ADC10ENC + ADC10SC;        // Sampling and conversion start

    __bis_SR_register(LPM0_bits + GIE);     // LPM0, ADC10_ISR will force exit
    __no_operation();                       // For debug only

    if (ADC_Result < 255)
      P1OUT &= ~BIT0;                       // Clear P1.0 LED off
    else
      P1OUT |= BIT0;                        // Set P1.0 LED on
  }
}

// ADC10 interrupt service routine
#pragma vector=ADC10_VECTOR
__interrupt void ADC10_ISR(void)
{
  switch(__even_in_range(ADC10IV,12))
  {
    case  0: break;                          // No interrupt
    case  2: break;                          // conversion result overflow
    case  4: break;                          // conversion time overflow
    case  6: break;                          // ADC10HI
    case  8: break;                          // ADC10LO
    case 10: break;                          // ADC10IN
    case 12: ADC_Result = ADC10MEM0;
             __bic_SR_register_on_exit(LPM0_bits);
             break;                          // Clear LPM0_bits from 0(SR)
    default: break;
  }
}

Upon debugging the contents of ADC_Result, I am getting a value of "613" although Analog input 0 is directly connected to the GND.  

Thank you very much in advance for your help.