Singular form just like the other error message.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index bbae2b1..5b30931 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -5954,7 +5954,7 @@
                 goto error;
             if (idx < 0) {
                 PyErr_SetString(PyExc_ValueError,
-                                "memos key must be positive integers.");
+                                "memo key must be positive integers.");
                 goto error;
             }
             if (_Unpickler_MemoPut(self, idx, value) < 0)