diff --git a/internal/aghalg/orderedmap.go b/internal/aghalg/orderedmap.go
index ec915e50cc7..5e15cf0733f 100644
--- a/internal/aghalg/orderedmap.go
+++ b/internal/aghalg/orderedmap.go
@@ -5,7 +5,7 @@ import (
 )
 
 // SortedMap is a map that keeps elements in order with internal sorting
-// function.
+// function.  Must be initialised by the [NewSortedMap].
 type SortedMap[K comparable, V any] struct {
 	vals map[K]V
 	cmp  func(a, b K) (res int)
@@ -23,36 +23,50 @@ func NewSortedMap[K comparable, V any](cmp func(a, b K) (res int)) SortedMap[K,
 	}
 }
 
-// Set adds val with key to the sorted map.
+// Set adds val with key to the sorted map.  It panics if the m is nil.
 func (m *SortedMap[K, V]) Set(key K, val V) {
+	m.vals[key] = val
+
 	i, has := slices.BinarySearchFunc(m.keys, key, m.cmp)
 	if has {
 		m.keys[i] = key
-		m.vals[key] = val
+	} else {
+		m.keys = slices.Insert(m.keys, i, key)
+	}
+}
 
+// Get returns val by key from the sorted map.
+func (m *SortedMap[K, V]) Get(key K) (val V, ok bool) {
+	if m == nil {
 		return
 	}
 
-	m.keys = slices.Insert(m.keys, i, key)
-	m.vals[key] = val
-}
+	val, ok = m.vals[key]
 
-// Get returns val by key from the sorted map.
-func (m *SortedMap[K, V]) Get(key K) (val V) {
-	return m.vals[key]
+	return val, ok
 }
 
 // Del removes the value by key from the sorted map.
 func (m *SortedMap[K, V]) Del(key K) {
-	i, has := slices.BinarySearchFunc(m.keys, key, m.cmp)
-	if has {
-		m.keys = slices.Delete(m.keys, i, i+1)
-		delete(m.vals, key)
+	if m == nil {
+		return
 	}
+
+	if _, has := m.vals[key]; !has {
+		return
+	}
+
+	delete(m.vals, key)
+	i, _ := slices.BinarySearchFunc(m.keys, key, m.cmp)
+	m.keys = slices.Delete(m.keys, i, i+1)
 }
 
 // Clear removes all elements from the sorted map.
 func (m *SortedMap[K, V]) Clear() {
+	if m == nil {
+		return
+	}
+
 	// TODO(s.chzhen):  Use built-in clear in Go 1.21.
 	m.keys = nil
 	m.vals = make(map[K]V)
@@ -61,6 +75,10 @@ func (m *SortedMap[K, V]) Clear() {
 // Range calls cb for each element of the map, sorted by m.cmp.  If cb returns
 // false it stops.
 func (m *SortedMap[K, V]) Range(cb func(K, V) (cont bool)) {
+	if m == nil {
+		return
+	}
+
 	for _, k := range m.keys {
 		if !cb(k, m.vals[k]) {
 			return
diff --git a/internal/aghalg/orderedmap_test.go b/internal/aghalg/orderedmap_test.go
index e7a5ae50cdc..46128ed0e5e 100644
--- a/internal/aghalg/orderedmap_test.go
+++ b/internal/aghalg/orderedmap_test.go
@@ -36,13 +36,18 @@ func TestNewSortedMap(t *testing.T) {
 
 		assert.Equal(t, letters, gotLetters)
 		assert.Equal(t, nums, gotNums)
-		assert.Equal(t, nums[0], m.Get(letters[0]))
+
+		n, ok := m.Get(letters[0])
+		assert.True(t, ok)
+		assert.Equal(t, nums[0], n)
 	})
 
 	t.Run("clear", func(t *testing.T) {
 		lastLetter := letters[len(letters)-1]
 		m.Del(lastLetter)
-		assert.Equal(t, 0, m.Get(lastLetter))
+
+		_, ok := m.Get(lastLetter)
+		assert.False(t, ok)
 
 		m.Clear()
 
@@ -56,3 +61,35 @@ func TestNewSortedMap(t *testing.T) {
 		assert.Len(t, gotLetters, 0)
 	})
 }
+
+func TestNewSortedMap_nil(t *testing.T) {
+	const (
+		key = "key"
+		val = "val"
+	)
+
+	var m SortedMap[string, string]
+
+	assert.Panics(t, func() {
+		m.Set(key, val)
+	})
+
+	assert.NotPanics(t, func() {
+		_, ok := m.Get(key)
+		assert.False(t, ok)
+	})
+
+	assert.NotPanics(t, func() {
+		m.Range(func(_, _ string) (cont bool) {
+			return true
+		})
+	})
+
+	assert.NotPanics(t, func() {
+		m.Del(key)
+	})
+
+	assert.NotPanics(t, func() {
+		m.Clear()
+	})
+}