diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -717,6 +717,9 @@ def revpair(repo, revs):
 
     if first is None:
         raise error.Abort(_('empty revision range'))
+    if (first == second and len(revs) >= 2
+        and not all(revrange(repo, [r]) for r in revs)):
+        raise error.Abort(_('empty revision on one side of range'))
 
     # if top-level is range expression, the result must always be a pair
     if first == second and len(revs) == 1 and not _pairspec(revs[0]):
diff --git a/tests/test-extdiff.t b/tests/test-extdiff.t
--- a/tests/test-extdiff.t
+++ b/tests/test-extdiff.t
@@ -70,6 +70,12 @@ Should diff cloned files directly:
   [1]
 #endif
 
+Specifying an empty revision should abort.
+
+  $ hg extdiff --patch --rev 'ancestor()' --rev 1
+  abort: empty revision on one side of range
+  [255]
+
 Test diff during merge:
 
   $ hg update -C 0