diff --git a/python/lammps.py b/python/lammps.py
index f8b29eeffc..836625188d 100644
--- a/python/lammps.py
+++ b/python/lammps.py
@@ -123,11 +123,14 @@ class lammps(object):
           # self.lmp = self.lib.lammps_open_no_mpi(0,None)
 
     else:
-      self.opened = 0
-      # magic to convert ptr to ctypes ptr
-      pythonapi.PyCObject_AsVoidPtr.restype = c_void_p
-      pythonapi.PyCObject_AsVoidPtr.argtypes = [py_object]
-      self.lmp = c_void_p(pythonapi.PyCObject_AsVoidPtr(ptr))
+      if isinstance(ptr,lammps):
+        # magic to convert ptr to ctypes ptr
+        pythonapi.PyCObject_AsVoidPtr.restype = c_void_p
+        pythonapi.PyCObject_AsVoidPtr.argtypes = [py_object]
+        self.lmp = c_void_p(pythonapi.PyCObject_AsVoidPtr(ptr))
+      else:
+        self.lmp = None
+        raise TypeError('Unsupported type passed as "ptr"')
 
   def __del__(self):
     if self.lmp and self.opened: self.lib.lammps_close(self.lmp)