diff --git a/lib/chibi/regexp.scm b/lib/chibi/regexp.scm
index 1ad20982..0b12654c 100644
--- a/lib/chibi/regexp.scm
+++ b/lib/chibi/regexp.scm
@@ -676,7 +676,9 @@
       (if (string? (car sre))
           (maybe-ci (string->char-set (car sre)))
           (case (car sre)
-            ((char-set) (maybe-ci (string->char-set (cadr sre))))
+            ((char-set) (if (null? (cddr sre))
+                            (maybe-ci (string->char-set (cadr sre)))
+                            (error "(char-set) takes only one char-set" sre)))
             ((/ char-range)
              (->cs
               `(or ,@(map (lambda (x)
@@ -689,10 +691,18 @@
             ((~ complement) (char-set-complement (->cs `(or ,@(cdr sre)))))
             ((- difference) (char-set-difference (->cs (cadr sre))
                                                  (->cs `(or ,@(cddr sre)))))
-            ((w/case) (sre->char-set (cadr sre) (flag-clear flags ~ci?)))
-            ((w/nocase) (sre->char-set (cadr sre) (flag-join flags ~ci?)))
-            ((w/ascii) (sre->char-set (cadr sre) (flag-join flags ~ascii?)))
-            ((w/unicode) (sre->char-set (cadr sre) (flag-clear flags ~ascii?)))
+            ((w/case) (if (null? (cddr sre))
+                          (sre->char-set (cadr sre) (flag-clear flags ~ci?))
+                          (error "w/case takes only one char-set" sre)))
+            ((w/nocase) (if (null? (cddr sre))
+                            (sre->char-set (cadr sre) (flag-join flags ~ci?))
+                            (error "w/nocase takes only one char-set" sre)))
+            ((w/ascii) (if (null? (cddr sre))
+                           (sre->char-set (cadr sre) (flag-join flags ~ascii?))
+                           (error "w/ascii takes only one char-set" sre)))
+            ((w/unicode) (if (null? (cddr sre))
+                             (sre->char-set (cadr sre) (flag-clear flags ~ascii?))
+                             (error "w/unicode takes only one char-set" sre)))
             (else (error "invalid sre char-set" sre)))))
      (else (error "invalid sre char-set" sre)))))