diff --git a/src/if_python3.c b/src/if_python3.c
index 77d7cd3..da0867b 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -1110,6 +1110,9 @@
     {
 	Py_ssize_t start, stop, step, slicelen;
 
+	if (CheckBuffer((BufferObject *) self))
+	    return NULL;
+
 	if (PySlice_GetIndicesEx((PyObject *)idx,
 	      (Py_ssize_t)((BufferObject *)(self))->buf->b_ml.ml_line_count+1,
 	      &start, &stop,
@@ -1139,6 +1142,9 @@
     {
 	Py_ssize_t start, stop, step, slicelen;
 
+	if (CheckBuffer((BufferObject *) self))
+	    return -1;
+
 	if (PySlice_GetIndicesEx((PyObject *)idx,
 	      (Py_ssize_t)((BufferObject *)(self))->buf->b_ml.ml_line_count+1,
 	      &start, &stop,