diff --git a/sys/kern/tty_ttydisc.c b/sys/kern/tty_ttydisc.c
--- a/sys/kern/tty_ttydisc.c
+++ b/sys/kern/tty_ttydisc.c
@@ -844,23 +844,25 @@
 					 */
 					ttyinq_write(&tp->t_inq, bytes,
 					    UTF8_STACKBUF, 0);
+					ttyinq_unputchar(&tp->t_inq);
 				} else {
 					/* Find codepoint and width. */
 					codepoint =
 					    teken_utf8_bytes_to_codepoint(bytes,
 						nb);
-					if (codepoint !=
-					    TEKEN_UTF8_INVALID_CODEPOINT) {
-						cwidth = teken_wcwidth(
-						    codepoint);
-					} else {
+					if (codepoint ==
+						TEKEN_UTF8_INVALID_CODEPOINT ||
+					    (cwidth = teken_wcwidth(
+						 codepoint)) == -1) {
 						/*
 						 * Place all bytes back into the
 						 * inq and fall back to
 						 * default behaviour.
 						 */
+						cwidth = 1;
 						ttyinq_write(&tp->t_inq, bytes,
 						    nb, 0);
+						ttyinq_unputchar(&tp->t_inq);
 					}
 				}
 				tp->t_column -= cwidth;
diff --git a/sys/teken/teken_wcwidth.h b/sys/teken/teken_wcwidth.h
--- a/sys/teken/teken_wcwidth.h
+++ b/sys/teken/teken_wcwidth.h
@@ -128,15 +128,32 @@
 teken_utf8_bytes_to_codepoint(uint8_t bytes[4], int nbytes)
 {
 
-  /* Check for malformed characters. */
-  if (__bitcount(bytes[0] & 0xf0) != nbytes)
+ /*
+  * Check for malformed characters by comparing 'nbytes'
+  * to the byte length of the character.
+  *
+  * The table in section 3 of RFC 3629 defines 4 different
+  * values indicating the length of a UTF-8 byte sequence.
+  *
+  * 0xxxxxxx -> 1 byte
+  * 110xxxxx -> 2 bytes
+  * 1110xxxx -> 3 bytes
+  * 11110xxx -> 4 bytes
+  *
+  * The length is determined by the higher-order bits in
+  * the leading octet (except in the first case, where an MSB
+  * of 0 means a byte length of 1). Here we flip the 4 upper
+  * bits and count the leading zeros using __builtin_clz()
+  * to determine the number of bytes.
+  */
+  if (__builtin_clz(~(bytes[0] & 0xf0) << 24) != nbytes)
     return (TEKEN_UTF8_INVALID_CODEPOINT);
 
   switch (nbytes) {
   case 1:
     return (bytes[0] & 0x7f);
   case 2:
-    return (bytes[0] & 0xf) << 6 | (bytes[1] & 0x3f);
+    return (bytes[0] & 0x1f) << 6 | (bytes[1] & 0x3f);
   case 3:
     return (bytes[0] & 0xf) << 12 | (bytes[1] & 0x3f) << 6 | (bytes[2] & 0x3f);
   case 4: