--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -53,7 +53,10 @@ def addremove(ui, repo, *pats, **opts):
     between 0 (disabled) and 100 (files must be identical) as its
     parameter.  Detecting renamed files this way can be expensive.
     """
-    sim = float(opts.get('similarity') or 0)
+    try:
+        sim = float(opts.get('similarity') or 0)
+    except ValueError:
+        raise util.Abort(_('similarity must be a number'))
     if sim < 0 or sim > 100:
         raise util.Abort(_('similarity must be between 0 and 100'))
     return cmdutil.addremove(repo, pats, opts, similarity=sim/100.)

--- a/tests/test-addremove-similar
+++ b/tests/test-addremove-similar
@@ -41,3 +41,9 @@ hg addremove -s50
 
 hg commit -m B
 
+echo % should all fail
+hg addremove -s foo
+hg addremove -s -1
+hg addremove -s 1e6
+
+true

--- a/tests/test-addremove-similar.out
+++ b/tests/test-addremove-similar.out
@@ -14,3 +14,7 @@ adding small-file
 removing large-file
 removing tiny-file
 recording removal of tiny-file as rename to small-file (82% similar)
+% should all fail
+abort: similarity must be a number
+abort: similarity must be between 0 and 100
+abort: similarity must be between 0 and 100