SQL Parameter Update Not Updating Changed Null/blank Values

I am attempting to update a sql db using the update and parameter code in VB.net 2003 through MSDE for a web application. It updates changed data OK, but if the textbox value is deleted, the code does not update the sql db. I am new to this, and I'm sure it is something simple. Here is some sample code.

SqlConnection1.Open()

strSQLu = "UPDATE table1 " _
& "SET Field1Tag = @Field1Tag, Field2Tag = @Field2Tag " _
& "WHERE (Field1Tag = @Field1Tag) "

cmdCategoriesUpdate.CommandText = strSQLu

With cmdCategoriesUpdate
.Parameters("@Field1Tag").Value = txtFld1.Text
.Parameters("@Field2Tag").Value = txtFld2.Text
End With
cmdCategoriesUpdate.ExecuteNonQuery()
SqlConnection1.Close()