diff --git a/python/ycm/completers/cs/cs_completer.py b/python/ycm/completers/cs/cs_completer.py
index 01a26108..fe5eff5a 100755
--- a/python/ycm/completers/cs/cs_completer.py
+++ b/python/ycm/completers/cs/cs_completer.py
@@ -154,7 +154,7 @@ class CsharpCompleter( Completer ):
     command = [ omnisharp, '-p', str( self._omnisharp_port ), '-s',
                 path_to_solutionfile ]
 
-    if utils.OnWindows():
+    if not utils.OnWindows():
       command.insert(0, 'mono')
 
     filename_format = os.path.join( utils.PathToTempDir(),