diff --git a/docs/dcc-validations.md b/docs/dcc-validations.md
index d1bb4f1..e3053b8 100644
--- a/docs/dcc-validations.md
+++ b/docs/dcc-validations.md
@@ -60,10 +60,12 @@ The rules treat a special missing as a value for some checks and as a missing va
 |ROUND|Ignored - ROUND only rounds values that are numbers, so a special missing is left as typed.|
 |HARDREGEX / SOFTREGEX|Checked against the pattern like any other value.  Unlike blanks and the plain `.`, special missings are **not** exempt.  See [Regex Rules](#regex-rules).|
 |SOFTSELECT|Passes - a soft dropdown never blocks a value.|
-|HARDSELECT|Fails - the strict membership test cannot match it (the dropdown list for a numeric column holds numbers).|
+|HARDSELECT|Checked against the dropdown list like any other value - it passes only if the list contains it.|
 |HARDFORMULA / SOFTFORMULA|The formula returns `#VALUE!` for that row instead of a number, and that is what gets submitted.|
 
-In short, a special missing is **not supported** in a column that carries NOTNULL, MINVAL, MAXVAL, HARDSELECT or a formula.  It is fine alongside the regex rules, a soft dropdown, and ROUND.
+In short, a special missing is **not supported** in a column that carries NOTNULL, MINVAL, MAXVAL or a formula.  It is fine alongside the regex rules, a soft dropdown, and ROUND.
+
+A primary key column is treated as NOT NULL whether or not the target table carries a physical constraint, and whether or not MPE_VALIDATIONS has a NOTNULL rule for it - a key identifies the row, so a blank and a special missing are both rejected there.
 
 One related gotcha: a range rule's own value must be a number.  Put a special missing in `MINVAL` and every cell in the column fails; put one in `MAXVAL` and every real number fails.