diff --git a/.gitignore b/.gitignore
index 7bcb779..83e970c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,4 +6,5 @@ build
 test/.*
 test/mongod.*
 test/fixtures/
-test/fixtures.replication/
\ No newline at end of file
+test/fixtures.replication/
+*mongo.log
\ No newline at end of file
diff --git a/oplogutils/_core.py b/oplogutils/_core.py
index f9f1e69..d6cd02b 100644
--- a/oplogutils/_core.py
+++ b/oplogutils/_core.py
@@ -62,8 +62,17 @@ def validate_options(self):
         pass
 
 
+    def connection(self):
+        return Connection(self.opts.host, self.opts.port)
+
+
+    def replication_enabled(self):
+        stat = self.connection().admin.command({ 'serverStatus': 1 })
+        return 'repl' in stat
+
+
     def db_local(self):
-        return Connection(self.opts.host, self.opts.port).local
+        return self.connection().local
 
 
     def oplog(self):
diff --git a/oplogutils/_trimmer.py b/oplogutils/_trimmer.py
index 528f9d3..ef271b5 100644
--- a/oplogutils/_trimmer.py
+++ b/oplogutils/_trimmer.py
@@ -71,6 +71,8 @@ def should_continue(self):
 
 
     def run(self):
+        self.assert_replication_off()
+
         q = CountQuery(start=self.opts.remove_after)
         self.affected_events = q.run(self.oplog())
 
@@ -86,5 +88,14 @@ def run(self):
             sys.exit(1)
 
 
+    def assert_replication_off(self):
+        if self.replication_enabled():
+            print """
+In order to use this command, the master database must be restarted without
+replication options. Back it up and restart it without the --master switch.
+"""
+            sys.exit(-1)
+
+
     def trim(self):
         pass
diff --git a/test/test_trimmer.py b/test/test_trimmer.py
index f1cf9a6..dceecb8 100644
--- a/test/test_trimmer.py
+++ b/test/test_trimmer.py
@@ -115,10 +115,9 @@ def test_always_yes_skips_confirm(self):
 
     def test_aborts_if_mongo_running_with_replication_options(self):
         s = self.trim(port=settings.MONGOD_REPLICATION_PORT, expect_code=-1)
-        self.assertTrue('with no replication options' in s)
+        self.assertTrue('without replication options' in s.replace('\n', ' '))
 
 
     def test_removing_events(self):
         pass
 
-