CF ERROR MSG: The value "" cannot be converted to a number

Hi All,

I need help in trying to figure out why a random error seems to happen. I have built a continuing education search and registration application. People will search for their workshops and then click a button to go and register/pay for the workshops. This has been live for a few months and the error below has happend a few times. It does not happen each time. It just seems to happen sporadically. I cannot figure out why most of the registrations will work and a few will not.

I am lost at why this happens every now and then. I think it is with the "TotalFees" variable.

Any suggestions would be wonderful! Thanks

ERROR:
MESSAGE The value "" cannot be converted to a number


CODE (IN THE BODY IS "ERROR OCCURS HERE ---->")
<cflock scope="session" type="readonly" timeout="5">
<CFQUERY NAME="versions" DATASOURCE="ceworkshops" DBTYPE="ODBC">
SELECT ID,Session_Number,Title,WorkshopDate,WorkshopTime,Member_Earlybird,Member_Advanced_Fee,Non_Member_Earlybird,Non_Member_Advanced_Fee,
Member_Onsite_Fee,Non_Member_Onsite_Fee,Credits
FROM celisting
WHERE id IN (#session.classes#)
ORDER BY session_number
</cfquery>
</cflock>

<p>Final Schedule For <cfoutput>#attributes.fname# #attributes.lname#</cfoutput></p>

<cfset totalFees = "0">

<cfoutput query="versions" group="workshopdate">
<p><span class="textbold">#workshopdate#</span></p>


<table>
<tr>
      <td>ID</td>
      <td>Title</td>
      <td>Credits</td>
      <td>Time</td>
      <td>Fee</td>
      
</tr>
<cfoutput>
<tr>
      <td>#session_number#</td>
      <td>#title#</td>
      <td>#credits#</td>
      <td>#workshoptime#</td>

      <cflock scope="session" type="readonly" timeout="5">

      <!---shows early bird rate--->
      <cfif #session.validMember# EQ "YES">
            <td>#DollarFormat(member_earlybird)#</td>
ERROR OCCURS HERE ---->            <cfset totalFees = #totalFees# + #member_earlybird#>
            
      <cfelse>
            <td>#DollarFormat(non_member_earlybird)#</td>
OR ERROR OCCURS HERE ---->            <cfset totalFees = #totalFees# + #non_member_earlybird#>
      </cfif>
      

      </cflock>
      
</tr>
</cfoutput>

</table>
</cfoutput>

<table width="100%">
<cfoutput>
<tr>
      <td>Total to be Charged to Credit Card:</td>
      <td>#DollarFormat(totalFees)#</td>
      <input type="hidden" name="totalfees" value="#totalfees#">
</tr>
</cfoutput>
</table>