When you are calling a method of a webservice and want to omit an unrequired numeric variable that has a default value set coldfusion will throw the following error:
The fault returned when invoking the web service operation is:<br>
<pre>'' java.lang.IllegalArgumentException</pre>
Example:
<cfinvoke
    webservice = "http://*.cfc?WSDL" 
    method="getFriendlyDay" 
    returnvariable="response"
    refreshWSDL="true"
>
        <cfinvokeargument name="dayNumber" omit="true"/>
</cfinvoke>
webservice component:
<cffunction name="getFriendlyDay" access="remote" returntype="any" output="no" description="get a friendly date from a number">   
        <cfargument name="dayNumber" type="numeric" required="no" default="0">
        ...
</cffunction>
				
                        
My solution to this is to just not omit the argument. Pass in the default value. I just wanted to record this in case someone else gets the same error. Thus far it has only occurred on numeric values.
Update:
I believe this probably relates to the bug outlined here: