ppolicy: invalid value for attributeType pwAttribute -- for "userPassword"

[Chris Shenton <chris.shenton@nasa.gov>]

I'm running 2.3.39 and using ppolicy to enforce our password policy. 
Got an LDIF file:

dn: cn=npg2810,ou=policies,dc=nasascience,dc=nasa,dc=gov
cn: npg2810
objectClass: top
objectClass: pwdPolicy
objectClass: organizationalRole
objectClass: pwdPolicyChecker
description: OpenLDAP ppolicy to implement NPG2810-like restrictions
pwdAttribute:                   userPassword

When I try to slapadd it, I get an error indicating it doesn't like 
"userPassword":

sudo sbin/slapadd -l ldifs/6_policies.ldif
str2entry: invalid value for attributeType pwdAttribute #0 (syntax 
1.3.6.1.4.1.1466.115.121.1.38)
slapadd: could not parse entry (line=51)

The ppolicy.schema file says:

#5.2.1  pwdAttribute
#
#   This holds the name of the attribute to which the password policy is
#   applied.  For example, the password policy may be applied to the
#   userPassword attribute.

attributetype ( 1.3.6.1.4.1.42.2.27.8.1.1
      NAME 'pwdAttribute'
      EQUALITY objectIdentifierMatch
      SYNTAX 1.3.6.1.4.1.1466.115.121.1.38 )

And my slapd.conf includes ppolicy.schema in addition to core.schema, 
cosine.schema, and inetorgperson.schema.

Oddly, replacing the "userPassword" value with a random number, like 
"42" or "3.14159" causes no error.

I cannot find userPassword defined in any of the schemas, tho it is 
commented out in core.schema.   If I uncomment it from core.schema it 
complains that it's a dupe:

sbin/slapadd -l ldifs/6_policies.ldif
schema/core.schema: line 244: Duplicate attributeType: "2.5.4.35"
slapadd: bad configuration file!


What am I missing?
Thanks.