diff --git a/cmd/rcsb/main.go b/cmd/rcsb/main.go
index be6254f..4391d60 100644
--- a/cmd/rcsb/main.go
+++ b/cmd/rcsb/main.go
@@ -98,6 +98,9 @@ func downloadAndParse(structureID string) ([]*pdb.Model, error) {
 		return nil, err
 	}
 	defer resp.Body.Close()
+	if code := resp.StatusCode; code != 200 {
+		return nil, fmt.Errorf("error %d while GETing %s", code, structureID)
+	}
 	r, err := gzip.NewReader(resp.Body)
 	if err != nil {
 		return nil, err
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..b90793c
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,11 @@
+module github.com/fogleman/ribbon
+
+go 1.17
+
+require (
+	github.com/fogleman/ease v0.0.0-20170301025033-8da417bf1776 // indirect
+	github.com/fogleman/fauxgl v0.0.0-20200818143847-27cddc103802 // indirect
+	github.com/fogleman/mc v0.0.0-20200516034030-c30b20ace55a // indirect
+	github.com/fogleman/simplify v0.0.0-20170216171241-d32f302d5046 // indirect
+	github.com/nfnt/resize v0.0.0-20180221191011-83c6a9932646 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..b90211f
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,10 @@
+github.com/fogleman/ease v0.0.0-20170301025033-8da417bf1776 h1:VRIbnDWRmAh5yBdz+J6yFMF5vso1It6vn+WmM/5l7MA=
+github.com/fogleman/ease v0.0.0-20170301025033-8da417bf1776/go.mod h1:9wvnDu3YOfxzWM9Cst40msBF1C2UdQgDv962oTxSuMs=
+github.com/fogleman/fauxgl v0.0.0-20200818143847-27cddc103802 h1:5vdq0jOnV15v1NdZbAcU+dIJ22rFgwaieiFewPvnKCA=
+github.com/fogleman/fauxgl v0.0.0-20200818143847-27cddc103802/go.mod h1:7f7F8EvO8MWvDx9sIoloOfZBCKzlWuZV/h3TjpXOO3k=
+github.com/fogleman/mc v0.0.0-20200516034030-c30b20ace55a h1:ITrp9WHc4bZYIMyXOL9o6M7v06cqZysOY8ToKfdJJgU=
+github.com/fogleman/mc v0.0.0-20200516034030-c30b20ace55a/go.mod h1:u8RqaMl2hN0XMPpPwzSOvUjYyyS2Rihdl1wqrxIspUk=
+github.com/fogleman/simplify v0.0.0-20170216171241-d32f302d5046 h1:n3RPbpwXSFT0G8FYslzMUBDO09Ix8/dlqzvUkcJm4Jk=
+github.com/fogleman/simplify v0.0.0-20170216171241-d32f302d5046/go.mod h1:KDwyDqFmVUxUmo7tmqXtyaaJMdGon06y8BD2jmh84CQ=
+github.com/nfnt/resize v0.0.0-20180221191011-83c6a9932646 h1:zYyBkD/k9seD2A7fsi6Oo2LfFZAehjjQMERAvZLEDnQ=
+github.com/nfnt/resize v0.0.0-20180221191011-83c6a9932646/go.mod h1:jpp1/29i3P1S/RLdc7JQKbRpFeM1dOBd8T9ki5s+AY8=