diff --git a/openupgradelib/openupgrade.py b/openupgradelib/openupgrade.py
index 680e3ae8..45d4df24 100644
--- a/openupgradelib/openupgrade.py
+++ b/openupgradelib/openupgrade.py
@@ -2883,16 +2883,19 @@ def lift_constraints(cr, table, column, cascade=False):
     Set cascade=True if other constraints depend on the one you want to delete,
     which ie is the case for primary keys.
     If everything went right, the constraints will be recreated."""
+    # PostgreSQL 18 stores NOT NULL as a named constraint in pg_constraint, and
+    # refuses to drop it while the column is still part of a primary key. Sort
+    # the primary key first so it is dropped before the not null constraint.
     cr.execute(
-        "select relname, array_agg(conname) from "
-        "(select t1.relname, c.conname "
+        "select relname, array_agg(conname order by (contype = 'p') desc) from "
+        "(select t1.relname, c.conname, c.contype "
         "from pg_constraint c "
         "join pg_attribute a "
         "on c.confrelid=a.attrelid and a.attnum=any(c.conkey) "
         "join pg_class t on t.oid=a.attrelid "
         "join pg_class t1 on t1.oid=c.conrelid "
         "where t.relname=%(table)s and attname=%(column)s "
-        "union select t.relname, c.conname "
+        "union select t.relname, c.conname, c.contype "
         "from pg_constraint c "
         "join pg_attribute a "
         "on c.conrelid=a.attrelid and a.attnum=any(c.conkey) "