Exactly! I get a similar, yet slightly different error using my general purpose routines:_$ (dec->string (base->dec "7ffffcf5c20" 16))
"879689834528"
_$ (dec->string (base->dec "1311768467294899695" 16))
"5627943617999796174848"
_$ (dec->string (base->dec "1234567890abcdef" 16))
"1311768467294899712"
_$ (dec->string (base->dec "fedcba0987654321" 16))
"18364757930599071744"
_$ (dec->string (base->dec "111111111111" 16))
"18764998447377"
_$ (dec->string (base->dec "ffffffffffff" 16))
"281474976710655"