diff --git a/Objects/classobject.c b/Objects/classobject.c
index ffd3f87..2415ed1 100644
--- a/Objects/classobject.c
+++ b/Objects/classobject.c
@@ -71,7 +71,11 @@
         }
         /* use borrowed references */
         newargs[0] = self;
-        memcpy(newargs + 1, args, totalargs * sizeof(PyObject *));
+        if (totalargs) { /* bpo-37138: if totalargs == 0, then args may be
+                          * NULL and calling memcpy() with a NULL pointer
+                          * is undefined behaviour. */
+            memcpy(newargs + 1, args, totalargs * sizeof(PyObject *));
+        }
         result = _PyObject_Vectorcall(func, newargs, nargs+1, kwnames);
         PyMem_Free(newargs);
     }