Is this a bug in the dialplan interpreter?

The code below works just fine as I've got the priority entered numerically (which it took me a while to figure out) it works fine.

[internal-calling-card]
exten => s,1,Answer
exten => s,2,Set(TIMEOUT(digit)=15)
exten => s,3,Set(TIMEOUT(response)=15)
exten => s/XXXXXXXXXX,4,DISA(no-password|outcall)
exten => s/YYYYYYYYYY,4,DISA(no-password|outcall)
exten => s/ZZZZZZZZZZ,4,DISA(no-password|outcall)
exten => s,4,Authenticate(ABCABC)
exten => s,n,DISA(no-password|outcall)

If I instead try to use the 'n' priority it fails.  Is this a bug, or am I reading the documentation wrong?

[internal-calling-card]
exten => s,1,Answer
exten => s,n,Set(TIMEOUT(digit)=15)
exten => s,n,Set(TIMEOUT(response)=15)
exten => s/XXXXXXXXXX,n,DISA(no-password|outcall)
exten => s/YYYYYYYYYY,n,DISA(no-password|outcall)
exten => s/ZZZZZZZZZZ,n,DISA(no-password|outcall)
exten => s,n,Authenticate(ABCABC)
exten => s,n,DISA(no-password|outcall)