diff --git a/nyquist/dspprims.lsp b/nyquist/dspprims.lsp
index 4015c8e59..c6750ee5a 100644
--- a/nyquist/dspprims.lsp
+++ b/nyquist/dspprims.lsp
@@ -306,13 +306,20 @@
 ; remember that snd-biquad uses the opposite sign convention for a_i's 
 ; than Matlab does.
 
+; convenient biquad: normalize a0, and use zero initial conditions.
 ; convenient biquad: normalize a0, and use zero initial conditions.
 (defun nyq:biquad (x b0 b1 b2 a0 a1 a2)
-  (if (< a0 1.0)
-      (error (format t "a0 < 1 (unstable parameter) in biquad~%")))
+  (if (<= a0 0.0)
+      (error (format nil "a0 < 0 (unstable parameter a0 = ~A) in biquad~%" a0)))
   (let ((a0r (/ 1.0 a0)))
-    (snd-biquad x (* a0r b0) (* a0r b1) (* a0r b2) 
-                  (* a0r a1) (* a0r a2) 0 0)))
+    (setf a1 (* a0r a1)
+          a2 (* a0r a2))
+    (if (or (<= a2 -1.0) (<= (- 1.0 a2) (abs a1)))
+        (error (format nil
+         "(a2 <= -1) or (1 - a2 <= |a1|) (~A a1 = ~A, a2 = ~A) in biquad~%"
+         "unstable parameters" a1 a2)))
+    (snd-biquad x (* a0r b0) (* a0r b1) (* a0r b2)
+                  a1 a2 0 0)))
 
 
 (defun biquad (x b0 b1 b2 a0 a1 a2)
diff --git a/plug-ins/SpectralEditShelves.ny b/plug-ins/SpectralEditShelves.ny
index 62b8ce6c3..ff6c4060a 100644
--- a/plug-ins/SpectralEditShelves.ny
+++ b/plug-ins/SpectralEditShelves.ny
@@ -30,12 +30,9 @@
 
 (defun wet (sig gain f0 f1)
   (cond
-    ((not f0) ;low-shelf
-      (if (< f1 (/ *sound-srate* 2.0))
-          (eq-lowshelf sig f1 gain)
-          (mult sig gain))) ;frequency above Nyquist so amplify full spectrum
-   ((not f1) (eq-highshelf sig f0 gain))
-   (t (mid-shelf sig f0 (validate f1) gain))))
+    ((not f0) (eq-lowshelf sig f1 gain))
+    ((not f1) (eq-highshelf sig f0 gain))
+    (t (mid-shelf sig f0 (validate f1) gain))))
 
 (defun result (sig)
   (let*
@@ -58,14 +55,23 @@
       (T (sum (prod env (wet sig control-gain f0 f1))
               (prod (diff 1.0 env) sig))))))
 
+;; Frequency selection must be between 0 Hz and Nyquist.
+(if (and (get '*selection* 'low-hz)
+         (<= (get '*selection* 'low-hz) 0))
+    (remprop '*selection* 'low-hz))
+(if (and (get '*selection* 'high-hz)
+         (>= (get '*selection* 'high-hz)(/ *sound-srate* 2)))
+    (remprop '*selection* 'high-hz))
+
 (cond
   ((not (get '*TRACK* 'VIEW)) ; 'View is NIL during Preview
       (setf p-err (format nil "This effect requires a frequency selection in the~%~
-                              'Spectrogram' or 'Spectrogram (log f)' track view.~%~%"))
+                              'Spectral Selection' or 'Spectral Selection log(f)'~%~
+                              track view.~%~%"))
       (catch 'error-message
         (multichan-expand #'result *track*)))
   ((string-not-equal (get '*TRACK* 'VIEW) "spectral"  :end1 8 :end2 8)
-      "Use this effect in the 'Spectral Selection'\nor 'Spectral Selection log(f)' view.")
+     "Use this effect in the 'Spectral Selection'\nor 'Spectral Selection log(f)' view.")
   (T  (setf p-err "")
       (if (= control-gain 0)  ; Allow dry preview
           "Gain is zero. Nothing to do."