Crystal reports return string or number

What I am trying to do is return a number or a string from CR formula. Something like

IF A/B != 0
return A/B
else
return "N/A"

Here is my attempt which unforunatelly does not compile - In this place a sequence is required (own translation from my native language), and points on the first line.

stringvar result := {report_lpb_requests_kpi;1.phone_requests};
IF {ps_hd_report_lpb_requests_kpi;1.all_requests} <> 0 THEN
    result := {report_lpb_requests_kpi;1.phone_requests} / {report_lpb_requests_kpi;1.all_requests} *100
ELSE
    result :='N/A'