diff --git a/test/programs/utf8.flan b/test/programs/utf8.flan
index 7e662a8..db5b269 100644
--- a/test/programs/utf8.flan
+++ b/test/programs/utf8.flan
@@ -28,6 +28,7 @@
 
 (defconst overlong2 [2 u8] [0xc0 0xaf])
 (defconst overlong3 [3 u8] [0xe0 0x80 0xaf])
+(defconst overlong4 [4 u8] [0xf0 0x80 0x80 0xaf])
 (defconst surrogate [3 u8] [0xed 0xa0 0x80])
 (defconst above-max [4 u8] [0xf4 0x90 0x80 0x80])
 (defconst lead-f5   [4 u8] [0xf5 0x80 0x80 0x80])
@@ -101,6 +102,9 @@
   (show-dec (slice lone-cont 0 1))    ; a continuation byte leading
   (show-dec (slice overlong2 0 2))    ; overlong "/"
   (show-dec (slice overlong3 0 3))    ; overlong "/" again, three bytes
+  (show-dec (slice overlong4 0 4))    ; and four. Added after a mutation run:
+                                      ; relaxing 0xf0's floor to 0x80 left the
+                                      ; whole suite green without this line.
   (show-dec (slice surrogate 0 3))    ; U+D800
   (show-dec (slice above-max 0 4))        ; U+110000
   (show-dec (slice lead-f5 0 4))          ; 0xf5 leads nothing
diff --git a/test/test_acceptance.ml b/test/test_acceptance.ml
index 1fb74f2..63e5ea8 100644
--- a/test/test_acceptance.ml
+++ b/test/test_acceptance.ml
@@ -904,7 +904,7 @@ ERR@7 unexpected token: not the kind the caller was reading
        shared. *)
     let utf8_out =
       "0/0/f 65/1/t 233/2/t 26085/3/t 128512/4/t \n\
-       0/1/f 0/1/f 0/1/f 0/1/f 0/1/f 0/1/f \n\
+       0/1/f 0/1/f 0/1/f 0/1/f 0/1/f 0/1/f 0/1/f \n\
        0/1/f 0/1/f 0/1/f 0/1/f \n\
        tft\n\
        0 3 8 13 3\n\