I am guessing this is because you are using a numeric expression and you have to have values for it to be interpreted.
You can use this to check for null values and have it use 0 instead.
Code:
var strTextBox1 as String 'although you may want to use integer here
var strTextBox2 as String
If IsNull(Me.TextBox1Name) Then
strTextBox1="0"
Else strTextBox1=Me.TextBox1Name
EndIf
If IsNull(Me.TextBox2Name) Then
strTextBox1="0"
Else strTextBox1=Me.TextBox2Name
EndIf
'Your expression would change to:
...>=strTextBox1 And <=strTextBox2...
FYI - my syntax isn't usually perfect so you will want to double check everything.