univ.i:

  Fix a bug in the storage of the SQL NULL value in 64-bit computers: the value code must fit in 32 bits

innobase/include/univ.i

@@ -204,8 +204,12 @@ headers may define 'bool' differently. Do not assume that 'bool' is a ulint! */
 #endif
 
 /* The following number as the length of a logical field means that the field
-has the SQL NULL as its value. */
-#define UNIV_SQL_NULL 	ULINT_UNDEFINED
+has the SQL NULL as its value. NOTE that because we assume that the length
+of a field is a 32-bit integer when we store it, for example, to an undo log
+on disk, we must have also this number fit in 32 bits, also in 64-bit
+computers! */
+
+#define UNIV_SQL_NULL 	ULINT32_UNDEFINED
 
 /* Lengths which are not UNIV_SQL_NULL, but bigger than the following
 number indicate that a field contains a reference to an externally