diff --git a/Include/pymem.h b/Include/pymem.h
index 671f967..0ab9463 100644
--- a/Include/pymem.h
+++ b/Include/pymem.h
@@ -83,14 +83,18 @@
  */
 
 #define PyMem_New(type, n) \
-	( (type *) PyMem_Malloc((n) * sizeof(type)) )
+  ( assert((n) <= PY_SIZE_MAX / sizeof(type)) , \
+	( (type *) PyMem_Malloc((n) * sizeof(type)) ) )
 #define PyMem_NEW(type, n) \
-	( (type *) PyMem_MALLOC((n) * sizeof(type)) )
+  ( assert((n) <= PY_SIZE_MAX / sizeof(type)) , \
+	( (type *) PyMem_MALLOC((n) * sizeof(type)) ) )
 
 #define PyMem_Resize(p, type, n) \
-	( (p) = (type *) PyMem_Realloc((p), (n) * sizeof(type)) )
+  ( assert((n) <= PY_SIZE_MAX / sizeof(type)) , \
+	( (p) = (type *) PyMem_Realloc((p), (n) * sizeof(type)) ) )
 #define PyMem_RESIZE(p, type, n) \
-	( (p) = (type *) PyMem_REALLOC((p), (n) * sizeof(type)) )
+  ( assert((n) <= PY_SIZE_MAX / sizeof(type)) , \
+	( (p) = (type *) PyMem_REALLOC((p), (n) * sizeof(type)) ) )
 
 /* PyMem{Del,DEL} are left over from ancient days, and shouldn't be used
  * anymore.  They're just confusing aliases for PyMem_{Free,FREE} now.
diff --git a/Include/pyport.h b/Include/pyport.h
index d579be2..9ddcbf3 100644
--- a/Include/pyport.h
+++ b/Include/pyport.h
@@ -117,6 +117,17 @@
 #   error "Python needs a typedef for Py_ssize_t in pyport.h."
 #endif
 
+/* Largest possible value of size_t.
+   SIZE_MAX is part of C99, so it might be defined on some
+   platforms. If it is not defined, (size_t)-1 is a portable
+   definition for C89, due to the way signed->unsigned 
+   conversion is defined. */
+#ifdef SIZE_MAX
+#define PY_SIZE_MAX SIZE_MAX
+#else
+#define PY_SIZE_MAX ((size_t)-1)
+#endif
+
 /* Largest positive value of type Py_ssize_t. */
 #define PY_SSIZE_T_MAX ((Py_ssize_t)(((size_t)-1)>>1))
 /* Smallest negative value of type Py_ssize_t. */