diff --git a/src/if_py_both.h b/src/if_py_both.h
index c44fc93..4cd3734 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -5619,6 +5619,7 @@
     int		status;
     PyObject	*pyfunc, *pymain;
     PyObject	*run_ret;
+    buf_T	*was_curbuf = curbuf;
 
     if (u_save((linenr_T)RangeStart - 1, (linenr_T)RangeEnd + 1) != OK)
     {
@@ -5671,7 +5672,9 @@
 #ifdef PY_CAN_RECURSE
 	*pygilstate = PyGILState_Ensure();
 #endif
-	if (!(line = GetBufferLine(curbuf, lnum)))
+	/* Check the line number, the command my have deleted lines. */
+	if (lnum > curbuf->b_ml.ml_line_count
+		|| !(line = GetBufferLine(curbuf, lnum)))
 	    goto err;
 	if (!(linenr = PyInt_FromLong((long) lnum)))
 	{
@@ -5684,9 +5687,19 @@
 	if (!ret)
 	    goto err;
 
+	/* Check that the command didn't switch to another buffer. */
+	if (curbuf != was_curbuf)
+	{
+	    Py_XDECREF(ret);
+	    goto err;
+	}
+
 	if (ret != Py_None)
 	    if (SetBufferLine(curbuf, lnum, ret, NULL) == FAIL)
+	    {
+		Py_XDECREF(ret);
 		goto err;
+	    }
 
 	Py_XDECREF(ret);
 	PythonIO_Flush();