I think the object is _not_ valid. Please look at the following example:
#include "frh.ch"
PROC Main
oFr := FrReportManager():New()
oFr:Create()
f := {{"t_id", "N", 10, 0}}
a := {}
AAdd (a, {100})
AAdd (a, {200})
AAdd (a, {300})
oA1 := oFr:AddArray ("niz1", a, f)
b := {}
AAdd (b, {100}) // (*)
AAdd (b, {200}) // (*)
f := {{"t_id", "N", 10, 0}} // (**)
oA2 := oFr:AddArray ("niz2", b, f)
oA2:SetMaster (oA1, {"t_id=t_id"})
oFr:DesignReport()
oFr:End()
RETURN
This application works correctly. But, if you delete the (*) lines (table oA2 is empty), receives the error "SetMaster detail - Detail field t_id not found".
BTW, seems to AddArray has a bug. Check the value of F before and after the call AddArray:
before:
f := {{"t_id", "N", 10, 0}}
after:
f := {"t_id", "N", 10, 0}