diff --git a/Modules/_bz2module.c b/Modules/_bz2module.c
index 661847ad26702e..9e85e0de42cd8d 100644
--- a/Modules/_bz2module.c
+++ b/Modules/_bz2module.c
@@ -129,6 +129,9 @@ typedef struct {
     PyThread_type_lock lock;
 } BZ2Decompressor;
 
+#define _BZ2Compressor_CAST(op)     ((BZ2Compressor *)(op))
+#define _BZ2Decompressor_CAST(op)   ((BZ2Decompressor *)(op))
+
 /* Helper functions. */
 
 static int
@@ -376,8 +379,9 @@ _bz2_BZ2Compressor_impl(PyTypeObject *type, int compresslevel)
 }
 
 static void
-BZ2Compressor_dealloc(BZ2Compressor *self)
+BZ2Compressor_dealloc(PyObject *op)
 {
+    BZ2Compressor *self = _BZ2Compressor_CAST(op);
     BZ2_bzCompressEnd(&self->bzs);
     if (self->lock != NULL) {
         PyThread_free_lock(self->lock);
@@ -388,7 +392,7 @@ BZ2Compressor_dealloc(BZ2Compressor *self)
 }
 
 static int
-BZ2Compressor_traverse(BZ2Compressor *self, visitproc visit, void *arg)
+BZ2Compressor_traverse(PyObject *self, visitproc visit, void *arg)
 {
     Py_VISIT(Py_TYPE(self));
     return 0;
@@ -680,8 +684,10 @@ _bz2_BZ2Decompressor_impl(PyTypeObject *type)
 }
 
 static void
-BZ2Decompressor_dealloc(BZ2Decompressor *self)
+BZ2Decompressor_dealloc(PyObject *op)
 {
+    BZ2Decompressor *self = _BZ2Decompressor_CAST(op);
+
     if(self->input_buffer != NULL) {
         PyMem_Free(self->input_buffer);
     }
@@ -697,7 +703,7 @@ BZ2Decompressor_dealloc(BZ2Decompressor *self)
 }
 
 static int
-BZ2Decompressor_traverse(BZ2Decompressor *self, visitproc visit, void *arg)
+BZ2Decompressor_traverse(PyObject *self, visitproc visit, void *arg)
 {
     Py_VISIT(Py_TYPE(self));
     return 0;