diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef74664..c79fd4f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -131,8 +131,8 @@ replacement, and each removed path, column or flag.
   and the CLI carries them all. `New` loads the modules `WithDataFS(modules...)` names and
   the folders `WithDataPath` names, and fails, naming `WithDataFS`, where no such option
   is given. A manifest's `reads` lists the modules its source reads by default, and a
-  reference to a path nothing provides fails, naming those modules or saying there are
-  none.
+  reference to a path nothing provides fails to load, naming every module the loaded
+  manifests read, or saying there are none.
 - A data source whose root holds a `.fejkdata.json` manifest with an `index` loads each
   category on the first call reaching it, as each shipped module does. Data failing to load,
   in `New` or on that first call, gives an error matching `ErrLoad`, and the CLI exits 1.
diff --git a/README.md b/README.md
index d3594d7..8f0296a 100644
--- a/README.md
+++ b/README.md
@@ -195,8 +195,9 @@ A manifest's `reads` lists, by any name its author chooses, every module the sou
 references read unless another source provides the same paths, directly or through
 another module; a manifest may carry it without an index. Nothing loads these modules:
 pass them yourself. A shipped module names a Go package: `sv_SE`'s manifest reads
-`github.com/larvit/fejkdata/data/geo/SE`. A reference to a path nothing provides fails,
-naming those modules, or saying the manifest names none.
+`github.com/larvit/fejkdata/data/geo/SE`. Data whose reference names a path nothing
+provides fails to load, naming every module the loaded manifests read, or saying they
+name none.
 
 Each locale carries `address`, `color`, `company`, `date`, `email`, `first-name`,
 `ip`, `last-name`, `person`, `phone`, `price`, `sentence`, `sex`, `time`, `url`,
diff --git a/data.go b/data.go
index 68dbc53..2cd962f 100644
--- a/data.go
+++ b/data.go
@@ -1,6 +1,7 @@
 package fejkdata
 
 import (
+	"slices"
 	"strings"
 
 	"github.com/larvit/fejkdata/internal/datafiles"
@@ -8,22 +9,31 @@ import (
 
 // loadSources merges sources in order, the last winning a clash. A source whose manifest
 // carries an index leaves its categories unloaded; every other source loads here, with
-// the unloaded categories it needs.
-func loadSources(sources []datafiles.Source) (folder, error) {
+// the unloaded categories it needs. It also returns, sorted, every module the sources'
+// manifests read by default.
+// docs/decisions.md#a-source-whose-manifest-carries-an-index-loads-each-category-on-first-reach-any-other-loads-in-new-with-what-it-reads
+func loadSources(sources []datafiles.Source) (folder, []string, error) {
 	root := folder{children: map[string]node{}}
+	var modules []string
 	for _, src := range sources {
-		if err := addSource(&root, src); err != nil {
-			return folder{}, err
+		m, err := src.Manifest()
+		if err != nil {
+			return folder{}, nil, err
+		}
+		if err := addSource(&root, src, m); err != nil {
+			return folder{}, nil, err
+		}
+		for _, module := range m.Reads {
+			if !slices.Contains(modules, module) {
+				modules = append(modules, module)
+			}
 		}
 	}
-	return root, loadReached(&root, categorySites(&root), nil)
+	slices.Sort(modules)
+	return root, modules, loadReached(&root, categorySites(&root), nil)
 }
 
-func addSource(root *folder, src datafiles.Source) error {
-	m, err := src.Manifest()
-	if err != nil {
-		return err
-	}
+func addSource(root *folder, src datafiles.Source, m datafiles.Manifest) error {
 	if m.Index != nil {
 		placeIndex(root, src, m.Index)
 		return nil
diff --git a/fejkdata.go b/fejkdata.go
index 4fcb36a..118c527 100644
--- a/fejkdata.go
+++ b/fejkdata.go
@@ -31,6 +31,8 @@ type Generator struct {
 	drawState *drawstate.State
 	root      folder // the categories as the node a path walks from, owned here so a walk allocates none
 	structs   map[reflect.Type]structResult
+	// defaultModules is every module the loaded sources' manifests read by default.
+	defaultModules []string
 }
 
 type config struct {
@@ -107,15 +109,15 @@ func New(opts ...Option) (*Generator, error) {
 	if c.err != nil {
 		return nil, c.err
 	}
-	root, err := loadSources(c.sources)
+	root, modules, err := loadSources(c.sources)
 	if err != nil {
-		return nil, fmt.Errorf("fejkdata: %w", loadError{err})
+		return nil, fmt.Errorf("fejkdata: %w", withDefaultModules(loadError{err}, modules))
 	}
 	seed, err := c.drawSeed()
 	if err != nil {
 		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
-	return &Generator{drawState: drawstate.New(seed), root: root}, nil
+	return &Generator{drawState: drawstate.New(seed), root: root, defaultModules: modules}, nil
 }
 
 // List returns the sorted dotted paths Fake renders: each category and every field,
diff --git a/index.go b/index.go
index fa1c67e..45b37c8 100644
--- a/index.go
+++ b/index.go
@@ -59,7 +59,7 @@ func (f *Generator) loadCallerPath(path string) (string, []string, error) {
 	}
 	if c, ok := categoryOn(&f.root, segs); ok && c.unloaded() {
 		if err := loadReached(&f.root, nil, []categoryAt{c}); err != nil {
-			return "", nil, fmt.Errorf("fejkdata: %w", loadError{err})
+			return "", nil, fmt.Errorf("fejkdata: %w", withDefaultModules(loadError{err}, f.defaultModules))
 		}
 	}
 	return path, segs, nil
diff --git a/inline.go b/inline.go
index a9e15b3..50ae382 100644
--- a/inline.go
+++ b/inline.go
@@ -37,7 +37,7 @@ func (f *Generator) NewTemplate(input string) (*Template, error) {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	if err := loadInline(&f.root, n, "template", false); err != nil {
-		return nil, f.noDataNote(fmt.Errorf("fejkdata: %w", err))
+		return nil, f.noDataNote(fmt.Errorf("fejkdata: %w", withDefaultModules(err, f.defaultModules)))
 	}
 	return &Template{g: f, n: n}, nil
 }
diff --git a/reference.go b/reference.go
index 60e7433..2977158 100644
--- a/reference.go
+++ b/reference.go
@@ -121,6 +121,19 @@ type noEntry struct{ seg string }
 
 func (e noEntry) Error() string { return fmt.Sprintf("no entry %q", e.seg) }
 
+// withDefaultModules adds to err, where data fails to load because a segment names nothing, the
+// modules the loaded sources' manifests read by default, or that they name none.
+func withDefaultModules(err error, modules []string) error {
+	var ne noEntry
+	if !errors.Is(err, ErrLoad) || !errors.As(err, &ne) {
+		return err
+	}
+	if len(modules) == 0 {
+		return fmt.Errorf("%w, and no loaded module names a module it reads by default", err)
+	}
+	return fmt.Errorf("%w; the loaded modules read by default %s", err, strings.Join(modules, ", "))
+}
+
 // noDataNote adds to err, where a segment names nothing and f holds no data at all, that no
 // data is loaded.
 func (f *Generator) noDataNote(err error) error {
diff --git a/resolve.go b/resolve.go
index 6ad41bd..5556502 100644
--- a/resolve.go
+++ b/resolve.go
@@ -4,8 +4,8 @@ import (
 	"fmt"
 )
 
-// templateSite is a template to resolve, with its folder and category, both empty for an inline
-// template, and eachNode's label for it.
+// templateSite is a template to resolve, with its folder, empty for an inline template, and
+// eachNode's label for it.
 type templateSite struct {
 	t      *template
 	folder []string
diff --git a/struct.go b/struct.go
index 61a41eb..cfe93fa 100644
--- a/struct.go
+++ b/struct.go
@@ -31,7 +31,7 @@ func (f *Generator) FakeStruct(v any) error {
 	defer f.mu.Unlock()
 	shape, err := f.structShapeOf(p.Elem().Type())
 	if err != nil {
-		return fmt.Errorf("fejkdata: %w", err)
+		return fmt.Errorf("fejkdata: %w", withDefaultModules(err, f.defaultModules))
 	}
 	shape.fill(f.drawState, p.Elem())
 	return nil