Use the MPD() accessor macro.
diff --git a/Modules/_decimal/_decimal.c b/Modules/_decimal/_decimal.c
index c4580a8..91aac87 100644
--- a/Modules/_decimal/_decimal.c
+++ b/Modules/_decimal/_decimal.c
@@ -3745,9 +3745,9 @@
 }
 
 static int
-nm_nonzero(PyDecObject *v)
+nm_nonzero(PyObject *v)
 {
-    return !mpd_iszero(v->dec);
+    return !mpd_iszero(MPD(v));
 }
 
 static PyObject *