diff --git a/docs/languages/r-provider.md b/docs/languages/r-provider.md
index 758a2bd89..3ca36597c 100644
--- a/docs/languages/r-provider.md
+++ b/docs/languages/r-provider.md
@@ -35,7 +35,7 @@ Imports:
 Calls:
 
 - Free calls, `obj$method()` calls (receiver typed as above), R6 `self$method()` calls, and calls inside native pipe (`|>`) chains.
-- A call through a name that a function's formal or one of its `for` variables binds (`function(data, transform) transform(data)`) gets no edge, not even to a top-level function of that name. The scope publishes those names as lexical names: they stop the lookup and the unique-name fallback. A formal with a constant default (`n = 2`, `x = NULL`, `s = "a"`) that the body never assigns is left out, because R skips a value that is not a function when it looks up a callee, so that call still reaches the function of that name (the rename provider reads it the same way); a formal that may hold a function, and a `for` variable, are not. A `for` loop outside every function binds nothing, and a variable assigned by `<-` in the body (`helper <- identity; helper(x)`) still does not shadow.
+- A call through a name that a function's formal or one of its `for` variables binds (`function(data, transform) transform(data)`) gets no edge, not even to a top-level function of that name. The scope publishes those names as lexical names: they stop the lookup and the unique-name fallback. A formal with a constant default (`n = 2`, `s = "a"`, `b = TRUE`) that the body never assigns is left out, because R skips a value that is not a function when it looks up a callee, so that call still reaches the function of that name (the rename provider reads it the same way); a formal that may hold a function, and a `for` variable, are not. A `NULL` default is not constant: it is the usual placeholder of an optional callback (`function(x, helper = NULL) helper(x)`), so that formal counts as one that may hold a function. A `for` loop outside every function binds nothing, and a variable assigned by `<-` in the body (`helper <- identity; helper(x)`) still does not shadow.
 - Named nested functions (closures) are indexed, and calls inside attribute to the nested function, which can shadow a top-level name. Top-level script calls outside any function are attributed to the File node.
 - `pkg::fn()` and `pkg:::fn()`, decided by the qualifier (see [Resolution model](#resolution-model)).
 
@@ -154,7 +154,7 @@ Not modelled:
   - a replacement function and its getter (`unsupported_occurrence`): a call of the name anywhere in an assignment target (`helper(z) <- 5`, `helper(z)$a <- 1`, `names(helper(z)) <- v`, the `->` forms), because R then calls `` `helper<-` ``; an old or new name ending in `<-`; and a string-defined setter (`"helper<-" <- function(...)`) of either name;
   - a new name that is already a global (`name_collision`): a top-level variable or function of that name in any indexed file (`assist <- 3`, a `for` variable, `3 -> assist`), a `<<-` or `->>` to it anywhere, or a function of that name in another file. Same-file function collisions were already refused by the shared engine; local variables, parameters and members of that name are allowed;
   - a new name that the sources already read as a value or call (`name_collision`): `sapply(x, paste)`, `Reduce(max, x)`, `x <- letters`, a default `f = assist`, a formula term, `assist[1]`, `assist(1)`. After the rename the function is the global of that name and would capture the read. A parameter of an enclosing function of that name covers a read, and covers a call only when it has no default. Not reads: bound names, argument names (`f(assist = 1)`), members (`x$assist`, `x@assist`) and qualified names (`pkg::assist`);
-  - a call of the name under a same-named parameter that may hold a function (`ambiguous_reference`): a function or computed default, or an assignment to the parameter in the body, because the call may reach the parameter or the function being renamed;
+  - a call of the name under a same-named parameter that may hold a function (`ambiguous_reference`): a function, `NULL` or computed default, or an assignment to the parameter in the body, because the call may reach the parameter or the function being renamed;
   - a function that is also assigned with `<<-` or `->>` anywhere in the indexed R sources (`ambiguous_reference`, as the top-level form already was): the global re-assignment is a second definition that a rename would leave behind;
   - a package function under `R/` that the index did not prove private, because `NAMESPACE` is not an indexed source and its entry would go stale. The rename tool reads the indexed `isExported` flag, which is false only for a function that a `NAMESPACE` file beside the package leaves out of `export()`, `exportPattern()` and `S3method()` (the third argument of `S3method(generic, class, method)` counts). A private package function is renamed; an exported one is refused; so is a function in a package with no `NAMESPACE` file, or in an `R/` folder that belongs to no package, where the index keeps the default public flag.
 - **Refused by the shared engine (fail-closed, not provider code):** S3 methods and any dotted function name (`area.circle` reads as an owner-qualified member, so `stale_graph`); S4 class names (`unsupported_occurrence`, no exact source-name span).
diff --git a/gitnexus/src/core/ingestion/languages/r/rename.ts b/gitnexus/src/core/ingestion/languages/r/rename.ts
index befb9a5df..bca9efc1b 100644
--- a/gitnexus/src/core/ingestion/languages/r/rename.ts
+++ b/gitnexus/src/core/ingestion/languages/r/rename.ts
@@ -98,7 +98,11 @@ export function registersS3Method(newName: string, defined: ReadonlySet<string>)
   return false;
 }
 
-/** Default values that can never be a function. */
+/**
+ * Default values that can never be a function. `NULL` is not one of them: it is the usual
+ * placeholder of an optional callback (`function(x, helper = NULL) helper(x)`), so the formal
+ * may hold a function when the caller supplies one.
+ */
 const CONSTANT_DEFAULTS: ReadonlySet<string> = new Set([
   'float',
   'integer',
@@ -106,7 +110,6 @@ const CONSTANT_DEFAULTS: ReadonlySet<string> = new Set([
   'string',
   'true',
   'false',
-  'null',
   'na',
   'inf',
   'nan',
diff --git a/gitnexus/test/fixtures/lang-resolution/r-formal-shadows/R/a.R b/gitnexus/test/fixtures/lang-resolution/r-formal-shadows/R/a.R
index a30c1647d..a926d0f04 100644
--- a/gitnexus/test/fixtures/lang-resolution/r-formal-shadows/R/a.R
+++ b/gitnexus/test/fixtures/lang-resolution/r-formal-shadows/R/a.R
@@ -17,4 +17,6 @@ plain <- function(data) transform(data)
 
 constant_default <- function(data, transform = 2) transform(data)
 
+null_default <- function(data, transform = NULL) transform(data)
+
 function_default <- function(data, validate = function(x) x) validate(data)
diff --git a/gitnexus/test/integration/resolvers/r-formal-shadows.test.ts b/gitnexus/test/integration/resolvers/r-formal-shadows.test.ts
index c1588e3f5..5176bc349 100644
--- a/gitnexus/test/integration/resolvers/r-formal-shadows.test.ts
+++ b/gitnexus/test/integration/resolvers/r-formal-shadows.test.ts
@@ -42,6 +42,10 @@ describe('R calls through a formal or a loop variable', () => {
     expect(targets('constant_default')).toEqual(['transform']);
   });
 
+  it('does not bind a call through a formal that defaults to NULL: it can hold a callback', () => {
+    expect(targets('null_default')).toEqual([]);
+  });
+
   it('does not bind a call through a formal that defaults to a function', () => {
     expect(targets('function_default')).toEqual([]);
   });
diff --git a/gitnexus/test/unit/r-rename-parameter-shadow.test.ts b/gitnexus/test/unit/r-rename-parameter-shadow.test.ts
index efb77fa7a..d1c2173bc 100644
--- a/gitnexus/test/unit/r-rename-parameter-shadow.test.ts
+++ b/gitnexus/test/unit/r-rename-parameter-shadow.test.ts
@@ -48,24 +48,14 @@ describe('rParameterKind', () => {
     expect(kindOf('use <- function(x, helper) helper(x)')).toBe('function');
   });
 
-  it.each([
-    '2',
-    '2L',
-    '1i',
-    '"s"',
-    'TRUE',
-    'FALSE',
-    'NULL',
-    'NA',
-    'NA_integer_',
-    'Inf',
-    'NaN',
-    '-1',
-  ])('treats the constant default %s as not a function', (constant) => {
-    expect(kindOf(`use <- function(x, helper = ${constant}) helper(x)`)).toBe('constant');
-  });
+  it.each(['2', '2L', '1i', '"s"', 'TRUE', 'FALSE', 'NA', 'NA_integer_', 'Inf', 'NaN', '-1'])(
+    'treats the constant default %s as not a function',
+    (constant) => {
+      expect(kindOf(`use <- function(x, helper = ${constant}) helper(x)`)).toBe('constant');
+    },
+  );
 
-  it.each(['function(z) z', '\\(z) z', 'identity', 'c(1, 2)', 'x + 1', 'list()'])(
+  it.each(['function(z) z', '\\(z) z', 'NULL', 'identity', 'c(1, 2)', 'x + 1', 'list()'])(
     'treats the default %s as possibly a function',
     (expression) => {
       expect(kindOf(`use <- function(x, helper = ${expression}) helper(x)`)).toBe('maybe');
@@ -107,6 +97,10 @@ describe('R rename: a call that a same-named parameter may or may not shadow', (
     );
   });
 
+  it('refuses a call under a parameter that defaults to NULL: it can hold a callback', () => {
+    expect(verdict('use <- function(x, helper = NULL) { helper(x) }')).toBe('ambiguous_reference');
+  });
+
   it('refuses a call under a parameter with a computed default', () => {
     expect(verdict('use <- function(x, helper = pick()) { helper(x) }')).toBe(
       'ambiguous_reference',
diff --git a/gitnexus/test/unit/scope-resolution/r/r-lexical-names.test.ts b/gitnexus/test/unit/scope-resolution/r/r-lexical-names.test.ts
index 4d593a2a8..588eb2e2e 100644
--- a/gitnexus/test/unit/scope-resolution/r/r-lexical-names.test.ts
+++ b/gitnexus/test/unit/scope-resolution/r/r-lexical-names.test.ts
@@ -31,9 +31,9 @@ describe('rFunctionLexicalNames', () => {
   });
 
   it('skips a formal with a constant default that the body never assigns', () => {
-    expect(namesOf('f <- function(a, n = 2, s = "x", b = TRUE, z = NULL, m = -1) 1')).toEqual([
-      'a',
-    ]);
+    expect(namesOf('f <- function(a, n = 2, s = "x", b = TRUE, m = -1) 1')).toEqual(['a']);
+    // NULL is the usual placeholder of an optional callback, so that formal may hold a function.
+    expect(namesOf('f <- function(a, z = NULL) 1')).toEqual(['a', 'z']);
     // Assigned in the body, the formal may hold a function again.
     expect(namesOf('f <- function(a, n = 2) { n <- identity; n(a) }')).toEqual(['a', 'n']);
     // A constant-default formal that a loop also binds is a loop variable.