Moc -r file.mo err, why?

       let a = Principal.fromActor(this);
        Debug.print(debug_show(id, Principal.toText(a)));

show_val: {
assetRequest = func;
authorize = func;
balanceOf = func;
balanceOfInsecure = func;
cycle = func;
getAuthorized = func;
getAuthorizedInsecure = func;
getContractInfo = func;
getContractInfoInsecure = func;
getEventCallbackStatus = func;
getMetadata = func;
getTotalMinted = func;
http_request = func;
http_request_streaming_callback = func;
init = func;
isAuthorized = func;
isAuthorizedInsecure = func;
listAssets = func;
mint = func;
ownerOf = func;
ownerOfInsecure = func;
postupgrade = func;
preupgrade = func;
queryProperties = func;
setEventCallback = func;
tokenChunkByID = func;
tokenChunkByIDInsecure = func;
tokensByID = func;
tokensByIDInsecure = func;
transfer = func;
updateContractOwners = func;
updateProperties = func;
wallet_receive = func;
writeStaged = func
} : Principal

        /Users/ipfs/.cache/dfinity/versions/0.7.2/base/Principal.mo:15.57-15.58: internal error, "Assert_failure mo_values/show.ml:88:4"

moc -r runs the interpreter which, unfortunately, does not implement all features of Motoko. I believe Principal.toText is probably one of them.