--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -45,7 +45,7 @@ def revrange(repo, revs):
     """Yield revision as strings from a list of revision specifications."""
 
     def revfix(repo, val, defval):
-        if not val and val != 0:
+        if not val and val != 0 and defval is not None:
             return defval
         return repo.changelog.rev(repo.lookup(val))
 

--- a/tests/test-log
+++ b/tests/test-log
@@ -76,3 +76,8 @@ hg log --follow-first
 
 echo % log -P 2
 hg log -P 2
+
+echo '% log -r ""'
+hg log -r ''
+
+exit 0

--- a/tests/test-log.out
+++ b/tests/test-log.out
@@ -185,3 +185,5 @@ user:        test
 date:        Thu Jan 01 00:00:01 1970 +0000
 summary:     b1
 
+% log -r ""
+abort: Ambiguous identifier!