Newbie: Error Checking in Dynamically typed Language

Hi all,
This is my first time working with a dynamically typed environment and my 
first instinct is to error check all arguments. For example suppose I want 
to create an object to represent an octet in an IP address (a number between 
0 and 256) I would do something like this:

Object subclass: #Octet
   instanceVariables: 'octetValue'
   ......

initialize
   super initialize.
   octetValue := 0.

setOctetValue: numBtw0And256
   octetValue = numBtw0And256        <-------Here's where I have a problem

How do I know the numBtw0And256 is actually an integer between 0 and 256 and 
not 'cat' or '3.14'?

Since I came from statically typed languages I feel like I writing a whole 
bunch of error  checking code along the lines of ifIsInteger: ... etc which 
doesn't seem to be in the spirit of Squeak.

What am I doing wrong?

Thanks

Deech

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