diff --git a/taskchampion/src/replica.rs b/taskchampion/src/replica.rs
index a3df48f7e..19937eb2c 100644
--- a/taskchampion/src/replica.rs
+++ b/taskchampion/src/replica.rs
@@ -48,7 +48,7 @@ impl Replica {
     }
 
     /// Add the given uuid to the working set, returning its index.
-    pub(crate) fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<u64> {
+    pub(crate) fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<usize> {
         self.taskdb.add_to_working_set(uuid)
     }
 
@@ -92,7 +92,7 @@ impl Replica {
     }
 
     /// Get an existing task by its working set index
-    pub fn get_working_set_task(&mut self, i: u64) -> Fallible<Option<Task>> {
+    pub fn get_working_set_task(&mut self, i: usize) -> Fallible<Option<Task>> {
         let working_set = self.taskdb.working_set()?;
         if (i as usize) < working_set.len() {
             if let Some(uuid) = working_set[i as usize] {
diff --git a/taskchampion/src/taskdb.rs b/taskchampion/src/taskdb.rs
index 2221326c7..a24ffff0b 100644
--- a/taskchampion/src/taskdb.rs
+++ b/taskchampion/src/taskdb.rs
@@ -153,13 +153,13 @@ impl TaskDB {
 
     /// Add the given uuid to the working set and return its index; if it is already in the working
     /// set, its index is returned.  This does *not* renumber any existing tasks.
-    pub fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<u64> {
+    pub fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<usize> {
         let mut txn = self.storage.txn()?;
         // search for an existing entry for this task..
         for (i, elt) in txn.get_working_set()?.iter().enumerate() {
             if *elt == Some(*uuid) {
                 // (note that this drops the transaction with no changes made)
-                return Ok(i as u64);
+                return Ok(i);
             }
         }
         // and if not found, add one
diff --git a/taskchampion/src/taskstorage/inmemory.rs b/taskchampion/src/taskstorage/inmemory.rs
index cdc376c5e..c438b548e 100644
--- a/taskchampion/src/taskstorage/inmemory.rs
+++ b/taskchampion/src/taskstorage/inmemory.rs
@@ -108,14 +108,13 @@ impl<'t> TaskStorageTxn for Txn<'t> {
         Ok(self.data_ref().working_set.clone())
     }
 
-    fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<u64> {
+    fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<usize> {
         let working_set = &mut self.mut_data_ref().working_set;
         working_set.push(Some(uuid.clone()));
-        Ok(working_set.len() as u64)
+        Ok(working_set.len())
     }
 
-    fn remove_from_working_set(&mut self, index: u64) -> Fallible<()> {
-        let index = index as usize;
+    fn remove_from_working_set(&mut self, index: usize) -> Fallible<()> {
         let working_set = &mut self.mut_data_ref().working_set;
         if index >= working_set.len() || working_set[index].is_none() {
             return Err(format_err!("No task found with index {}", index));
diff --git a/taskchampion/src/taskstorage/kv.rs b/taskchampion/src/taskstorage/kv.rs
index 8cbe033b7..f7cd0cc4b 100644
--- a/taskchampion/src/taskstorage/kv.rs
+++ b/taskchampion/src/taskstorage/kv.rs
@@ -302,7 +302,7 @@ impl<'t> TaskStorageTxn for Txn<'t> {
         Ok(res)
     }
 
-    fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<u64> {
+    fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<usize> {
         let working_set_bucket = self.working_set_bucket();
         let numbers_bucket = self.numbers_bucket();
         let kvtxn = self.kvtxn();
@@ -323,10 +323,11 @@ impl<'t> TaskStorageTxn for Txn<'t> {
             NEXT_WORKING_SET_INDEX.into(),
             Msgpack::to_value_buf(next_index + 1)?,
         )?;
-        Ok(next_index)
+        Ok(next_index as usize)
     }
 
-    fn remove_from_working_set(&mut self, index: u64) -> Fallible<()> {
+    fn remove_from_working_set(&mut self, index: usize) -> Fallible<()> {
+        let index = index as u64;
         let working_set_bucket = self.working_set_bucket();
         let numbers_bucket = self.numbers_bucket();
         let kvtxn = self.kvtxn();
diff --git a/taskchampion/src/taskstorage/mod.rs b/taskchampion/src/taskstorage/mod.rs
index 699b59ff2..8c37e934b 100644
--- a/taskchampion/src/taskstorage/mod.rs
+++ b/taskchampion/src/taskstorage/mod.rs
@@ -80,10 +80,10 @@ pub trait TaskStorageTxn {
 
     /// Add a task to the working set and return its (one-based) index.  This index will be one greater
     /// than the highest used index.
-    fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<u64>;
+    fn add_to_working_set(&mut self, uuid: &Uuid) -> Fallible<usize>;
 
     /// Remove a task from the working set.  Other tasks' indexes are not affected.
-    fn remove_from_working_set(&mut self, index: u64) -> Fallible<()>;
+    fn remove_from_working_set(&mut self, index: usize) -> Fallible<()>;
 
     /// Clear all tasks from the working set in preparation for a garbage-collection operation.
     fn clear_working_set(&mut self) -> Fallible<()>;