diff --git a/README.md b/README.md
index eba9970..dc3646f 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ don't expect commits, docs, or comments to be any verbose.
 — [bs.py](/lib/bs.py)
 
 * biquad butterworth/chebyshev filters [(via DSPFilters)][dspf]
-— [butterworth.py](/lib/butterworth.py)
+— [nsf.py](/lib/nsf.py)
 
 * bilinear transformation: s-plane to z-plane
 — [planes.py](/lib/planes.py)
diff --git a/autoupdate b/autoupdate
index 1318b16..83bd402 100755
--- a/autoupdate
+++ b/autoupdate
@@ -1,7 +1,7 @@
 #!/usr/bin/env zsh
 set -e
 
-rm -r lib
+[ -d lib ] && rm -r lib
 dropbox_uploader -q download py/lib >/dev/null
 
 if [ ! -d .git ]; then
diff --git a/lib/data.py b/lib/data.py
index e1533cc..45e7cc3 100644
--- a/lib/data.py
+++ b/lib/data.py
@@ -3,7 +3,7 @@ from .bq import *
 
 import numpy as np
 
-# as calculated by LPB in butterworth.py
+# as calculated by LPB in nsf.py
 _bq2a = 1/.76536686473017945
 _bq2b = 1/1.8477590650225735
 _bq2a_bw = isqrt2/_bq2a