As is mentioned in the description of the Set! statement, Scheme variables can change type depending on what is being bound to the variable. For example, you can create a variable as an integer, but then assign it a Boolean value like #f at a later time. Once the variable is set to #f, it ceases being an integer and becomes Boolean instead. Since Scheme offers a lot of flexibility in variable manipulation, it is important to double check your syntax each time you set a variable to a certain value. Small unnoticed mistakes, such as a period in the middle of a number, can change the type of the variable without you noticing. These kinds of problems will cause more issues down the line as it becomes harder to locate your mistake.