diff --git a/crates/fidc-core/src/numeric_factors.rs b/crates/fidc-core/src/numeric_factors.rs
index 81b985d..320339e 100644
--- a/crates/fidc-core/src/numeric_factors.rs
+++ b/crates/fidc-core/src/numeric_factors.rs
@@ -19,6 +19,7 @@ pub struct NumericFactorMap {
 
 #[derive(Clone)]
 enum Storage {
+    Empty,
     Owned(Vec<(CompactString, f64)>),
     Shared(SharedRow),
 }
@@ -50,12 +51,13 @@ fn compact_key(key: Cow<'static, str>) -> CompactString {
 impl NumericFactorMap {
     pub const fn new() -> Self {
         Self {
-            storage: Storage::Owned(Vec::new()),
+            storage: Storage::Empty,
         }
     }
 
     pub fn len(&self) -> usize {
         match &self.storage {
+            Storage::Empty => 0,
             Storage::Owned(entries) => entries.len(),
             Storage::Shared(row) => row.len(),
         }
@@ -65,6 +67,7 @@ impl NumericFactorMap {
     }
     pub fn clear(&mut self) {
         match &mut self.storage {
+            Storage::Empty => {},
             Storage::Owned(entries) => entries.clear(),
             Storage::Shared(_) => *self = Self::new(),
         }
@@ -77,6 +80,7 @@ impl NumericFactorMap {
 
     pub(crate) fn has_normalized_finite_entries(&self) -> bool {
         match &self.storage {
+            Storage::Empty => true,
             Storage::Owned(entries) => entries.iter().all(|(name, value)| {
                 normalized_name(name) && value.is_finite()
             }),
@@ -85,18 +89,23 @@ impl NumericFactorMap {
     }
 
     fn owned_entries(&mut self) -> &mut Vec<(CompactString, f64)> {
+        if matches!(self.storage, Storage::Empty) {
+            self.storage = Storage::Owned(Vec::new());
+        }
         if matches!(self.storage, Storage::Shared(_)) {
             let entries = self.iter().map(|(key, value)| (key.clone(), *value)).collect();
             self.storage = Storage::Owned(entries);
         }
         match &mut self.storage {
             Storage::Owned(entries) => entries,
+            Storage::Empty => unreachable!("empty storage was initialized"),
             Storage::Shared(_) => unreachable!("shared row was materialized"),
         }
     }
 
     pub fn get(&self, key: &str) -> Option<&f64> {
         match &self.storage {
+            Storage::Empty => None,
             Storage::Owned(entries) => entries
                 .binary_search_by(|(name, _)| name.as_str().cmp(key))
                 .ok()
@@ -106,7 +115,9 @@ impl NumericFactorMap {
     }
 
     pub fn get_mut(&mut self, key: &str) -> Option<&mut f64> {
-        if !self.contains_key(key) {
+        if matches!(self.storage, Storage::Empty)
+            || matches!(self.storage, Storage::Shared(_)) && !self.contains_key(key)
+        {
             return None;
         }
         let entries = self.owned_entries();
@@ -145,7 +156,9 @@ impl NumericFactorMap {
     }
 
     pub fn remove(&mut self, key: &str) -> Option<f64> {
-        if !self.contains_key(key) {
+        if matches!(self.storage, Storage::Empty)
+            || matches!(self.storage, Storage::Shared(_)) && !self.contains_key(key)
+        {
             return None;
         }
         let entries = self.owned_entries();
@@ -161,6 +174,7 @@ impl NumericFactorMap {
 
     pub fn iter(&self) -> Iter<'_> {
         match &self.storage {
+            Storage::Empty => Iter(IterStorage::Owned([].iter())),
             Storage::Owned(entries) => Iter(IterStorage::Owned(entries.iter())),
             Storage::Shared(row) => Iter(IterStorage::Shared(row.iter())),
         }
@@ -228,6 +242,7 @@ impl IntoIterator for NumericFactorMap {
     type IntoIter = std::vec::IntoIter<Self::Item>;
     fn into_iter(self) -> Self::IntoIter {
         match self.storage {
+            Storage::Empty => Vec::new().into_iter(),
             Storage::Owned(entries) => entries.into_iter(),
             Storage::Shared(row) => row.iter().map(|(key, value)| (key.clone(), *value))
                 .collect::<Vec<_>>().into_iter(),
@@ -255,7 +270,11 @@ impl FromIterator<(CompactString, f64)> for NumericFactorMap {
                 false
             }
         });
-        Self { storage: Storage::Owned(entries) }
+        if entries.is_empty() {
+            Self::new()
+        } else {
+            Self { storage: Storage::Owned(entries) }
+        }
     }
 }
 impl Extend<(Cow<'static, str>, f64)> for NumericFactorMap {
@@ -367,7 +386,7 @@ mod tests {
         let normalized = rows.iter().map(NumericFactorMap::has_normalized_finite_entries).collect::<Vec<_>>();
         assert_eq!(NumericFactorMap::share_rows(rows.iter_mut()), rows.len());
         for (index, row) in rows.iter().enumerate() {
-            assert!(matches!(row.storage, Storage::Shared(_)));
+            assert!(row.is_empty() || matches!(row.storage, Storage::Shared(_)));
             assert_eq!(row_bits(row), before[index]);
             assert_eq!(serde_json::to_string(row).unwrap(), serialized[index]);
             assert_eq!(row.has_normalized_finite_entries(), normalized[index]);
diff --git a/crates/fidc-core/src/numeric_factors/shared_rows.rs b/crates/fidc-core/src/numeric_factors/shared_rows.rs
index 263bdd3..c8c16f4 100644
--- a/crates/fidc-core/src/numeric_factors/shared_rows.rs
+++ b/crates/fidc-core/src/numeric_factors/shared_rows.rs
@@ -100,7 +100,9 @@ pub(super) fn share<'a>(rows: impl IntoIterator<Item = &'a mut NumericFactorMap>
         return 0;
     }
     let Some(owned_bytes) = rows.iter().try_fold(0usize, |sum, row| {
-        let Storage::Owned(entries) = &row.storage else { return None };
+        let Storage::Owned(entries) = &row.storage else {
+            return matches!(row.storage, Storage::Empty).then_some(sum);
+        };
         sum.checked_add(entries.capacity().checked_mul(std::mem::size_of::<(CompactString, f64)>())?)
     }) else { return 0 };
     let mut names = BTreeMap::<&str, &CompactString>::new();
@@ -166,7 +168,11 @@ pub(super) fn share<'a>(rows: impl IntoIterator<Item = &'a mut NumericFactorMap>
     });
     let count = rows.len();
     for (index, row) in rows.into_iter().enumerate() {
-        row.storage = Storage::Shared(SharedRow { data: Arc::clone(&data), index });
+        row.storage = if data.lengths[index] == 0 {
+            Storage::Empty
+        } else {
+            Storage::Shared(SharedRow { data: Arc::clone(&data), index })
+        };
     }
     count
 }