Empty User Agent string checking with RegEx fails

Why does this not work for the User Agent empty string?

<cfif REFindNoCase("^$", CGI.HTTP_USER_AGENT)>

   <cfoutput>

      "#CGI.HTTP_USER_AGENT#"

   </cfoutput>

</cfif>

For obvious reasons this works as an alternative.

<cfif ! len(CGI.HTTP_USER_AGENT)>

   <cfoutput>

      "#CGI.HTTP_USER_AGENT#"

   </cfoutput>

</cfif>

However, I'm not really interested in a workaround but the reason why the firt RegEx approach fails.

Testing conducted with FF Add-On "User Agent Switcher" I'm able to impersonate a browser that is void of a UA.

Other info:

Adobe ColdFusion 9.0.1

Server 2008 R2 64 bit

Testing code from within application.cfm