diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -536,11 +536,14 @@ def buildstate(repo, dest, src, base, de
 
     if src:
         commonbase = repo[src].ancestor(repo[dest])
-        samebranch = repo[src].branch() == repo[dest].branch()
         if commonbase == repo[src]:
             raise util.Abort(_('source is ancestor of destination'))
-        if samebranch and commonbase == repo[dest]:
-            raise util.Abort(_('source is descendant of destination'))
+        if commonbase == repo[dest]:
+            samebranch = repo[src].branch() == repo[dest].branch()
+            if samebranch and repo[src] in repo[dest].children():
+                raise util.Abort(_('source is a child of destination'))
+            # rebase on ancestor, force detach
+            detach = True
         source = repo[src].rev()
         if detach:
             # We need to keep track of source's ancestors up to the common base
diff --git a/tests/test-rebase-detach.t b/tests/test-rebase-detach.t
--- a/tests/test-rebase-detach.t
+++ b/tests/test-rebase-detach.t
@@ -281,3 +281,25 @@ Rebasing across null as ancestor
   |/
   o  0: 'A'
   
+
+  $ hg rebase -d 5 -s 7
+  saved backup bundle to $TESTTMP/a5/.hg/strip-backup/13547172c9c0-backup.hg
+  $ hg tglog
+  @  8: 'D'
+  |
+  o  7: 'C'
+  |
+  | o  6: 'B'
+  |/
+  o  5: 'extra branch'
+  
+  o  4: 'H'
+  |
+  | o  3: 'G'
+  |/|
+  o |  2: 'F'
+  | |
+  | o  1: 'E'
+  |/
+  o  0: 'A'
+  
diff --git a/tests/test-rebase-parameters.t b/tests/test-rebase-parameters.t
--- a/tests/test-rebase-parameters.t
+++ b/tests/test-rebase-parameters.t
@@ -51,7 +51,7 @@ These fail:
   $ cd a1
 
   $ hg rebase -s 8 -d 7
-  abort: source is descendant of destination
+  abort: source is a child of destination
   [255]
 
   $ hg rebase --continue --abort
diff --git a/tests/test-rebase-scenario-global.t b/tests/test-rebase-scenario-global.t
--- a/tests/test-rebase-scenario-global.t
+++ b/tests/test-rebase-scenario-global.t
@@ -212,7 +212,7 @@ G onto F - rebase onto an ancestor:
   $ cd a7
 
   $ hg rebase -s 6 -d 5
-  abort: source is descendant of destination
+  abort: source is a child of destination
   [255]
 
 F onto G - rebase onto a descendant:
@@ -248,3 +248,25 @@ F onto G - rebase onto a descendant:
   nothing to rebase
   [1]
 
+C onto A - rebase onto an ancestor:
+
+  $ hg rebase -d 0 -s 2
+  saved backup bundle to $TESTTMP/a7/.hg/strip-backup/5fddd98957c8-backup.hg
+  $ hg tglog
+  @  7: 'D'
+  |
+  o  6: 'C'
+  |
+  | o  5: 'H'
+  | |
+  | | o  4: 'G'
+  | |/|
+  | o |  3: 'F'
+  |/ /
+  | o  2: 'E'
+  |/
+  | o  1: 'B'
+  |/
+  o  0: 'A'
+  
+