Index: Modules/dbmmodule.c
===================================================================
--- Modules/dbmmodule.c	(revision 84317)
+++ Modules/dbmmodule.c	(working copy)
@@ -168,12 +168,18 @@
 dbm_contains(register dbmobject *dp, PyObject *v)
 {
     datum key, val;
+    Py_ssize_t dsize;
 
-    if (PyString_AsStringAndSize(v, (char **)&key.dptr,
-                                 (Py_ssize_t *)&key.dsize)) {
+    if (PyString_AsStringAndSize(v, (char **)&key.dptr, &dsize)) {
         return -1;
     }
 
+    /* Coerce from Py_ssize_t down to int: */
+    if (dsize > INT_MAX) {
+        return -1;
+    }
+    key.dsize = dsize;
+
     /* Expand check_dbmobject_open to return -1 */
     if (dp->di_dbm == NULL) {
         PyErr_SetString(DbmError, "DBM object has already been closed");