diff --git a/S06-signature/arity.t b/S06-signature/arity.t
index 6f8fd9c727..ace48963dd 100644
--- a/S06-signature/arity.t
+++ b/S06-signature/arity.t
@@ -2,7 +2,7 @@ use v6;
 
 use Test;
 
-plan 52;
+plan 54;
 
 # L<S06/Required parameters/method:>
 sub a_zero  ()           { };
@@ -138,4 +138,12 @@ dies-ok { EVAL("a_zero( 'hello', 'world' )") }, 'no matching sub signature';
 is (-> *@a { }).count, Inf, 'slurpy positional causes infinite count';  #OK not used
 is (-> *%a { }).count, 0,   'slurpy named causes no count change';      #OK not used
 
+# RT #78240
+{
+    sub a ($a, $b, $c) { 42 }
+    my &b = &a.assuming(1);
+    is &a.arity, 3, 'arity of original sub is 3';
+    is &b.arity, 2, '.assuming(1) reduces arity by 1';
+}
+
 # vim: ft=perl6