diff --git a/collects/net/head-unit.ss b/collects/net/head-unit.ss
index b9b9404..efecd46 100644
--- a/collects/net/head-unit.ss
+++ b/collects/net/head-unit.ss
@@ -17,6 +17,9 @@
       (define re:continue (regexp "^[ \t\v]"))
       
       (define (validate-header s)
+	(let ([m (regexp-match #rx"[^\0-\277]" s)])
+	  (when m
+	    (error 'validate-header "non-Latin-1 character in string: ~a" (car m))))
 	(let ([len (string-length s)])
 	  (let loop ([offset 0])
 	    (cond
diff --git a/collects/net/mime-unit.ss b/collects/net/mime-unit.ss
index d1de387..31673c9 100644
--- a/collects/net/mime-unit.ss
+++ b/collects/net/mime-unit.ss
@@ -214,8 +214,8 @@
 
       (define mime-analyze
         (opt-lambda (input (part #f))
-          (let* ((iport (if (string? input)
-                            (open-input-string input)
+          (let* ((iport (if (bytes? input)
+                            (open-input-bytes input)
                             input))
                  (headers (get-headers iport))
                  (msg (if part
diff --git a/collects/net/smtp-unit.ss b/collects/net/smtp-unit.ss
index a3d7e1f..5414c19 100644
--- a/collects/net/smtp-unit.ss
+++ b/collects/net/smtp-unit.ss
@@ -40,7 +40,12 @@
 
       (define (protect-line l)
 	;; If begins with a dot, add one more
-	(if (or (string=? "" l) (not (char=? #\. (string-ref l 0))))
+	(if (or (equal? l #"")
+		(equal? l "")
+		(and (string? l)
+		     (not (char=? #\. (string-ref l 0))))
+		(and (bytes? l)
+		     (not (= (char->integer #\.) (bytes-ref l 0)))))
 	    l
 	    (string-append "." l)))