diff --git a/basis/grouping/grouping-tests.factor b/basis/grouping/grouping-tests.factor
index c91e5a56d6..52b436507e 100644
--- a/basis/grouping/grouping-tests.factor
+++ b/basis/grouping/grouping-tests.factor
@@ -1,5 +1,5 @@
 USING: grouping tools.test kernel sequences arrays
-math ;
+math accessors ;
 IN: grouping.tests
 
 [ { 1 2 3 } 0 group ] must-fail
@@ -12,6 +12,15 @@ IN: grouping.tests
     >array
 ] unit-test
 
+[ 0 ] [ { } 2 <clumps> length ] unit-test
+[ 0 ] [ { 1 } 2 <clumps> length ] unit-test
+[ 1 ] [ { 1 2 } 2 <clumps> length ] unit-test
+[ 2 ] [ { 1 2 3 } 2 <clumps> length ] unit-test
+
+[ 1 ] [ V{ } 2 <clumps> 0 over set-length seq>> length ] unit-test
+[ 2 ] [ V{ } 2 <clumps> 1 over set-length seq>> length ] unit-test
+[ 3 ] [ V{ } 2 <clumps> 2 over set-length seq>> length ] unit-test
+
 [ { { 1 2 } { 2 3 } } ] [ { 1 2 3 } 2 <sliced-clumps> [ >array ] map ] unit-test
 
 [ f ] [ [ { } { } "Hello" ] all-equal? ] unit-test
diff --git a/basis/grouping/grouping.factor b/basis/grouping/grouping.factor
index 83579d2beb..8a39a5d5cf 100644
--- a/basis/grouping/grouping.factor
+++ b/basis/grouping/grouping.factor
@@ -46,7 +46,7 @@ M: abstract-groups group@
 TUPLE: abstract-clumps < chunking-seq ;
 
 M: abstract-clumps length
-    [ seq>> length ] [ n>> ] bi - 1 + ; inline
+    [ seq>> length 1 + ] [ n>> ] bi [-] ; inline
 
 M: abstract-clumps set-length
     [ n>> + 1 - ] [ seq>> ] bi set-length ; inline