diff --git a/doc/guide/admin/slapdconf2.sdf b/doc/guide/admin/slapdconf2.sdf
index c5c82e1307..764763f4e3 100644
--- a/doc/guide/admin/slapdconf2.sdf
+++ b/doc/guide/admin/slapdconf2.sdf
@@ -740,7 +740,7 @@ H4: olcBkMdbIdlExp <exponent>
 
 Specify a power of 2 for the maximum size of an index slot.
 The default is 16, yielding a maximum slot size of 2^16 or 65536.
-The specified value must be in the range of 16-31.
+The specified value must be in the range of 16-30.
 
 This setting helps with the case where certain search filters are
 slow to return results due to an index slot having collapsed to a
diff --git a/doc/guide/admin/slapdconfig.sdf b/doc/guide/admin/slapdconfig.sdf
index 2ebc671ad4..3156b521e0 100644
--- a/doc/guide/admin/slapdconfig.sdf
+++ b/doc/guide/admin/slapdconfig.sdf
@@ -595,7 +595,7 @@ H4: idlexp <exponent>
 
 Specify a power of 2 for the maximum size of an index slot.
 The default is 16, yielding a maximum slot size of 2^16 or 65536.
-The specified value must be in the range of 16-31.
+The specified value must be in the range of 16-30.
 
 This setting helps with the case where certain search filters are
 slow to return results due to an index slot having collapsed to a
diff --git a/doc/man/man5/slapd-mdb.5 b/doc/man/man5/slapd-mdb.5
index 9e1bb1ab59..75f9335914 100644
--- a/doc/man/man5/slapd-mdb.5
+++ b/doc/man/man5/slapd-mdb.5
@@ -27,7 +27,7 @@ come before any subsequent "backend" or "database" lines.
 Specify a power of 2 for the maximum size of an index slot.
 The default is 16, yielding a maximum slot size of 2^16 or 65536.
 Once set, this option applies to every \fBmdb\fP database instance.
-The specified value must be in the range of 16-31.
+The specified value must be in the range of 16-30.
 .LP
 
 These
diff --git a/servers/slapd/back-mdb/config.c b/servers/slapd/back-mdb/config.c
index c0c4b4013c..380e4ff5eb 100644
--- a/servers/slapd/back-mdb/config.c
+++ b/servers/slapd/back-mdb/config.c
@@ -167,7 +167,8 @@ mdb_bk_cfg( ConfigArgs *c )
 		mdb_idl_reset();
 		c->bi->bi_private = 0;
 	} else {
-		if ( c->value_int >= MDB_IDL_LOGN && c->value_int < sizeof(int) * CHAR_BIT ) {
+		/* with 32 bit ints, db_size max is 2^30 and um_size max is 2^31 */
+		if ( c->value_int >= MDB_IDL_LOGN && ( c->value_int < sizeof(int) * CHAR_BIT - 1 )) {
 			MDB_idl_logn = c->value_int;
 			mdb_idl_reset();
 			c->bi->bi_private = (void *)8;	/* non-NULL to show we're using it */