OID could not be expanded: "IA5String"

["B M" <bobtuffet@hotmail.com>]

i just included the messageRecipient.schema file in my slapd.conf in order 
to use the messageRecipient and messageStoreRecipient object classes.  now 
when i start up my slapd i get the following error:

/etc/openldap/schema/messageRecipient.schema: line 78: OID could not be 
expanded: "IA5String"

line 78 is the first occurence of IA5String in the schema file that isn't 
commented out.

segment of the messageRecipient.schema file:

attributetype ( 2.16.840.1.113730.3.1.24
	NAME 'mailRoutingAddress'
	DESC 'RFC822 address to use when routing messages to the SMTP MTA of this 
recipient'
	EQUALITY caseIgnoreIA5Match
	SUBSTR caseIgnoreIA5SubstringsMatch
	SYNTAX IA5String
	SINGLE-VALUE
	)

could i just go through the file and replace IA5String with 
1.3.6.1.4.1.1466.115.121.1.26?  to be quite honest i have no idea why this 
error is caused or why OpenLDAP isn't recognizing IA5String or what needs to 
be set up to expand OIDs in the first place.