Error in Do Loop

Parsing code.                           0.02
Compiling code.                         Error
Error compiling program.
Error description: Object reference not set to an instance of an object.
Occurred on line: 428
Do 
Word: do

Sub btnLeft_Click
   btnStop_Click
   NewServiceMessage("Turning Left " , "", 1, 1000)
   
   Dim lCurrentHeading As Long 
   Dim lTargetHeading As Long
   Dim lAdjustedHeading As Long    
   
   lTargetHeading = lCurrentHeading - lTurnDegrees    
   If lTargetHeading < 0 Then 
      lTargetHeading = lTargetHeading + 360
   End If
   
   Do 
      lCurrentHeading = Heading
      
      If lTargetHeading > (360 - lTurnDegrees) AND lCurrentHeading < lTurnDegrees Then
         lCurrentHeading = lCurrentHeading + 360
      End If
      
      pinMotor4.DutyCycle = CurrentDutyCycle
      pinMotor3.DutyCycle = CurrentDutyCycle
   Loop While (lCurrentHeading < lTargetHeading)
   
End Sub