diff --git a/src/if_python3.c b/src/if_python3.c
index 55779e7..99781fc 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -294,7 +294,7 @@
 static PyObject* (*py3_PyTuple_GetItem)(PyObject *, Py_ssize_t);
 static int (*py3_PyMapping_Check)(PyObject *);
 static PyObject* (*py3_PyMapping_Keys)(PyObject *);
-static int (*py3_PySlice_GetIndicesEx)(PyObject *r, Py_ssize_t length,
+static int (*py3_PySlice_GetIndicesEx)(PySliceObject *r, Py_ssize_t length,
 		     Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step,
 		     Py_ssize_t *slicelen);
 static PyObject* (*py3_PyErr_NoMemory)(void);
@@ -1190,7 +1190,7 @@
 	if (CheckBuffer((BufferObject *) self))
 	    return NULL;
 
-	if (PySlice_GetIndicesEx((PyObject *)idx,
+	if (PySlice_GetIndicesEx((PySliceObject *)idx,
 	      (Py_ssize_t)((BufferObject *)(self))->buf->b_ml.ml_line_count,
 	      &start, &stop,
 	      &step, &slicelen) < 0)
@@ -1222,7 +1222,7 @@
 	if (CheckBuffer((BufferObject *) self))
 	    return -1;
 
-	if (PySlice_GetIndicesEx((PyObject *)idx,
+	if (PySlice_GetIndicesEx((PySliceObject *)idx,
 	      (Py_ssize_t)((BufferObject *)(self))->buf->b_ml.ml_line_count,
 	      &start, &stop,
 	      &step, &slicelen) < 0)
@@ -1306,7 +1306,7 @@
     {
 	Py_ssize_t start, stop, step, slicelen;
 
-	if (PySlice_GetIndicesEx((PyObject *)idx,
+	if (PySlice_GetIndicesEx((PySliceObject *)idx,
 		((RangeObject *)(self))->end-((RangeObject *)(self))->start+1,
 		&start, &stop,
 		&step, &slicelen) < 0)
@@ -1333,7 +1333,7 @@
     {
 	Py_ssize_t start, stop, step, slicelen;
 
-	if (PySlice_GetIndicesEx((PyObject *)idx,
+	if (PySlice_GetIndicesEx((PySliceObject *)idx,
 		((RangeObject *)(self))->end-((RangeObject *)(self))->start+1,
 		&start, &stop,
 		&step, &slicelen) < 0)