diff --git a/hgext/fsmonitor/pywatchman/__init__.py b/hgext/fsmonitor/pywatchman/__init__.py
--- a/hgext/fsmonitor/pywatchman/__init__.py
+++ b/hgext/fsmonitor/pywatchman/__init__.py
@@ -825,7 +825,7 @@
             p = subprocess.Popen(cmd, **args)
 
         except OSError as e:
-            raise WatchmanError('"watchman" executable not in PATH (%s)', e)
+            raise WatchmanError('"watchman" executable not in PATH (%s)' % e)
 
         stdout, stderr = p.communicate()
         exitcode = p.poll()