diff --git a/Include/asdl.h b/Include/asdl.h
index 7c3b6d1..495153c 100644
--- a/Include/asdl.h
+++ b/Include/asdl.h
@@ -31,11 +31,13 @@
 #define asdl_seq_GET(S, I) (S)->elements[(I)]
 #define asdl_seq_LEN(S) ((S) == NULL ? 0 : (S)->size)
 #ifdef Py_DEBUG
-#define asdl_seq_SET(S, I, V) { \
-        int _asdl_i = (I); \
-        assert((S) && _asdl_i < (S)->size); \
+#define asdl_seq_SET(S, I, V) \
+    do { \
+        Py_ssize_t _asdl_i = (I); \
+        assert((S) != NULL); \
+        assert(_asdl_i < (S)->size); \
         (S)->elements[_asdl_i] = (V); \
-}
+    } while (0)
 #else
 #define asdl_seq_SET(S, I, V) (S)->elements[I] = (V)
 #endif