diff --git a/CHANGELOG.md b/CHANGELOG.md
index f79657a9..b0f02c18 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ it releases that version ([RELEASING.md](https://github.com/energy-models/mathsp
 
 ## Upcoming version
 
+- feat(language): a parameter value that is null or NaN is refused when the data is attached ([#788](https://github.com/energy-models/mathspec/pull/788))
 - fix(language): a where string names several columns in at's over= and into=, as an expression does ([#782](https://github.com/energy-models/mathspec/pull/782))
 - fix(language): a file whose terms read each other's sums is refused at load ([#780](https://github.com/energy-models/mathspec/pull/780))
 - fix(language): dual(c) is the rate at which the optimal objective rises with the right side of c, so an equality has a sign too ([#751](https://github.com/energy-models/mathspec/pull/751))
diff --git a/docs/reference/language/absence.md b/docs/reference/language/absence.md
index ecb57e7d..45fa3f36 100644
--- a/docs/reference/language/absence.md
+++ b/docs/reference/language/absence.md
@@ -36,6 +36,8 @@ The [grammar](expressions.md#where-strings) says what a `where:` may hold.
 Nothing else creates absence. **A missing parameter row is not absence.** It
 reads as the value that contributes nothing: `0` as a coefficient, and `false`
 in a `where`.
+A missing row is the only gap a parameter has: a null or NaN value is
+[refused](declarations.md#parameters) when the data is attached.
 
 Where no such value exists, loading is refused. There are four such positions:
 a divisor, a `bounds:` entry, the whole constant side of a comparison, and a
diff --git a/docs/reference/language/declarations.md b/docs/reference/language/declarations.md
index 07ef0595..d32c8b4d 100644
--- a/docs/reference/language/declarations.md
+++ b/docs/reference/language/declarations.md
@@ -38,6 +38,9 @@ The column has to match the `dtype`:
 | `bool`   | a boolean column                  | `1` and `0` are not booleans. Cast the column |
 | `str`    | a string column                   |                                               |
 
+**A null or NaN value is refused** when the data is attached. A coordinate
+with no value has no row. `inf` and `-inf` are values.
+
 Only `float` and `int` are values. A `str` parameter is a label and a `bool`
 parameter is a mask: each selects rows in a
 [`where`](expressions.md#where-strings), and writing either as a coefficient,
diff --git a/src/mathspec/program.py b/src/mathspec/program.py
index ad882ec9..f32a9955 100644
--- a/src/mathspec/program.py
+++ b/src/mathspec/program.py
@@ -591,7 +591,9 @@ class ParameterDeclaration:
 
     ``dtype`` is what the declaration claims the values are, and a consumer
     attaching data refuses a column that is not it — so the *declaration* is
-    what is read, rather than whatever the column happens to hold.
+    what is read, rather than whatever the column happens to hold. It
+    refuses a null or NaN value too: a coordinate with no value has no row,
+    and ``inf`` is a value.
     """
 
     dims: tuple[str, ...]