diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index d1696991683..3bac4db7422 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -240,9 +240,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       provided none of the records in the table contain a
       <literal>NULL</literal> value for the column.  Ordinarily this is
       checked during the <literal>ALTER TABLE</literal> by scanning the
-      entire table; however, if a valid <literal>CHECK</literal> constraint is
-      found which proves no <literal>NULL</literal> can exist, then the
-      table scan is skipped.
+      entire table, unless <literal>NOT VALID</literal> is specified;
+      however, if a valid <literal>CHECK</literal> constraint exists
+      (and is not dropped in the same command) which proves no
+      <literal>NULL</literal> can exist, then the table scan is skipped.
       If a column has an invalid not-null constraint,
       <literal>SET NOT NULL</literal> validates it.
      </para>