nargs=N with N integer implies that the variable is a list.

As conv.converter.convert takes a string as an input, we need to
subscript the list to its first item.

flatlatex/bin/flatlatex.py

@@ -41,7 +41,7 @@ def do_convert(arguments):
         arguments.disallow_combinings,
     )
 
-    return converter.convert(arguments.input)
+    return converter.convert(arguments.input[0])
 
 
 def main():