diff --git a/wallet/src/address.rs b/wallet/src/address.rs
index dbf6587..9b3c51c 100644
--- a/wallet/src/address.rs
+++ b/wallet/src/address.rs
@@ -74,6 +74,10 @@ impl AddressEntry {
     }
 
     pub fn is_custodian(&self) -> bool {
+        self.role == Role::Custodian
+    }
+
+    pub fn is_controller_or_custodian(&self) -> bool {
         self.role == Role::Controller || self.role == Role::Custodian
     }
 }
@@ -136,6 +140,12 @@ impl AddressBook {
         self.find(principal).map_or(false, |e| e.is_controller())
     }
 
+    #[inline]
+    pub fn is_controller_or_custodian(&self, principal: &Principal) -> bool {
+        self.find(principal)
+            .map_or(false, |e| e.is_controller_or_custodian())
+    }
+
     #[inline]
     pub fn custodians(&self) -> impl Iterator<Item = &AddressEntry> {
         self.iter().filter(|e| e.is_custodian())
diff --git a/wallet/src/lib.did b/wallet/src/lib.did
index 3d604aa..8e92b44 100644
--- a/wallet/src/lib.did
+++ b/wallet/src/lib.did
@@ -56,17 +56,17 @@ type AddressEntry = record {
   role: Role;
 };
 
-type ResultCreate = variant {
+type WalletResultCreate = variant {
   Ok : record { canister_id: principal };
   Err: text;
 };
 
-type ResultSend = variant {
+type WalletResult = variant {
   Ok : null;
   Err : text;
 };
 
-type ResultCall = variant {
+type WalletResultCall = variant {
   Ok : record { return: blob };
   Err : text;
 };
@@ -91,22 +91,22 @@ service : {
   // Controller Management
   get_controllers: () -> (vec principal) query;
   add_controller: (principal) -> ();
-  remove_controller: (principal) -> ();
+  remove_controller: (principal) -> (WalletResult);
 
   // Custodian Management
   get_custodians: () -> (vec principal) query;
   authorize: (principal) -> ();
-  deauthorize: (principal) -> ();
+  deauthorize: (principal) -> (WalletResult);
 
   // Cycle Management
   wallet_balance: () -> (record { amount: nat64 }) query;
-  wallet_send: (record { canister: principal; amount: nat64 }) -> (ResultSend);
+  wallet_send: (record { canister: principal; amount: nat64 }) -> (WalletResult);
   wallet_receive: () -> ();  // Endpoint for receiving cycles.
 
   // Managing canister
-  wallet_create_canister: (CreateCanisterArgs) -> (ResultCreate);
+  wallet_create_canister: (CreateCanisterArgs) -> (WalletResultCreate);
 
-  wallet_create_wallet: (CreateCanisterArgs) -> (ResultCreate);
+  wallet_create_wallet: (CreateCanisterArgs) -> (WalletResultCreate);
 
   wallet_store_wallet_wasm: (record {
     wasm_module: blob;
@@ -118,12 +118,12 @@ service : {
     method_name: text;
     args: blob;
     cycles: nat64;
-  }) -> (ResultCall);
+  }) -> (WalletResultCall);
 
   // Address book
   add_address: (address: AddressEntry) -> ();
   list_addresses: () -> (vec AddressEntry) query;
-  remove_address: (address: principal) -> ();
+  remove_address: (address: principal) -> (WalletResult);
 
   // Events
   get_events: (opt record { from: opt nat32; to: opt nat32; }) -> (vec Event) query;
diff --git a/wallet/src/lib.rs b/wallet/src/lib.rs
index 7c23454..4377c26 100644
--- a/wallet/src/lib.rs
+++ b/wallet/src/lib.rs
@@ -87,7 +87,7 @@ fn post_upgrade() {
 /***************************************************************************************************
  * Wallet Name
  **************************************************************************************************/
-#[query(guard = "is_custodian")]
+#[query(guard = "is_custodian_or_controller")]
 fn name() -> Option<String> {
     storage::get::<WalletName>().0.clone()
 }
@@ -108,7 +108,7 @@ include!(concat!(env!("OUT_DIR"), "/http_request.rs"));
  **************************************************************************************************/
 
 /// Get the controller of this canister.
-#[query(guard = "is_custodian")]
+#[query(guard = "is_custodian_or_controller")]
 fn get_controllers() -> Vec<&'static Principal> {
     storage::get_mut::<AddressBook>()
         .controllers()
@@ -125,14 +125,25 @@ fn add_controller(controller: Principal) {
 
 /// Remove a controller. This is equivalent to moving the role to a regular user.
 #[update(guard = "is_controller")]
-fn remove_controller(controller: Principal) {
-    let book = storage::get_mut::<AddressBook>();
+fn remove_controller(controller: Principal) -> Result<(), String> {
+    if !storage::get::<AddressBook>().is_controller(&controller) {
+        return Err(format!(
+            "Cannot remove {} because it is not a controller.",
+            controller.to_text()
+        ));
+    }
+    if storage::get::<AddressBook>().controllers().count() > 1 {
+        let book = storage::get_mut::<AddressBook>();
 
-    if let Some(mut entry) = book.take(&controller) {
-        entry.role = Role::Contact;
-        book.insert(entry);
+        if let Some(mut entry) = book.take(&controller) {
+            entry.role = Role::Contact;
+            book.insert(entry);
+        }
+        update_chart();
+        Ok(())
+    } else {
+        Err("The wallet must have at least one controller.".to_string())
     }
-    update_chart();
 }
 
 /***************************************************************************************************
@@ -140,7 +151,7 @@ fn remove_controller(controller: Principal) {
  **************************************************************************************************/
 
 /// Get the custodians of this canister.
-#[query(guard = "is_custodian")]
+#[query(guard = "is_custodian_or_controller")]
 fn get_custodians() -> Vec<&'static Principal> {
     storage::get::<AddressBook>()
         .custodians()
@@ -157,13 +168,21 @@ fn authorize(custodian: Principal) {
 
 /// Deauthorize a custodian.
 #[update(guard = "is_controller")]
-fn deauthorize(custodian: Principal) {
-    remove_address(custodian);
-    update_chart();
+fn deauthorize(custodian: Principal) -> Result<(), String> {
+    if storage::get::<AddressBook>().is_custodian(&custodian) {
+        remove_address(custodian)?;
+        update_chart();
+        Ok(())
+    } else {
+        Err(format!(
+            "Cannot deauthorize {} as it is not a custodian.",
+            custodian.to_text()
+        ))
+    }
 }
 
 mod wallet {
-    use crate::{events, is_custodian};
+    use crate::{events, is_custodian_or_controller};
     use ic_cdk::export::candid::{CandidType, Nat};
     use ic_cdk::export::Principal;
     use ic_cdk::{api, caller, id, storage};
@@ -185,7 +204,7 @@ mod wallet {
     }
 
     /// Return the cycle balance of this canister.
-    #[query(guard = "is_custodian", name = "wallet_balance")]
+    #[query(guard = "is_custodian_or_controller", name = "wallet_balance")]
     fn balance() -> BalanceResult {
         BalanceResult {
             amount: api::canister_balance() as u64,
@@ -193,7 +212,7 @@ mod wallet {
     }
 
     /// Send cycles to another canister.
-    #[update(guard = "is_custodian", name = "wallet_send")]
+    #[update(guard = "is_custodian_or_controller", name = "wallet_send")]
     async fn send(args: SendCyclesArgs) -> Result<(), String> {
         match api::call::call_with_payment(
             args.canister.clone(),
@@ -276,7 +295,7 @@ mod wallet {
         canister_id: Principal,
     }
 
-    #[update(guard = "is_custodian", name = "wallet_create_canister")]
+    #[update(guard = "is_custodian_or_controller", name = "wallet_create_canister")]
     async fn create_canister(args: CreateCanisterArgs) -> Result<CreateResult, String> {
         let create_result = create_canister_call(args).await?;
 
@@ -429,7 +448,7 @@ mod wallet {
         Ok(())
     }
 
-    #[update(guard = "is_custodian", name = "wallet_create_wallet")]
+    #[update(guard = "is_custodian_or_controller", name = "wallet_create_wallet")]
     async fn create_wallet(args: CreateCanisterArgs) -> Result<CreateResult, String> {
         let wallet_bytes = storage::get::<super::WalletWASMBytes>();
         let wasm_module = match &wallet_bytes.0 {
@@ -503,7 +522,7 @@ mod wallet {
     }
 
     /// Forward a call to another canister.
-    #[update(guard = "is_custodian", name = "wallet_call")]
+    #[update(guard = "is_custodian_or_controller", name = "wallet_call")]
     async fn call(args: CallCanisterArgs) -> Result<CallResult, String> {
         if api::id() == caller() {
             return Err("Attempted to call forward on self. This is not allowed. Call this method via a different custodian.".to_string());
@@ -550,16 +569,23 @@ fn add_address(address: AddressEntry) {
     update_chart();
 }
 
-#[query(guard = "is_custodian")]
+#[query(guard = "is_custodian_or_controller")]
 fn list_addresses() -> Vec<&'static AddressEntry> {
     storage::get::<AddressBook>().iter().collect()
 }
 
 #[update(guard = "is_controller")]
-fn remove_address(address: Principal) {
-    storage::get_mut::<AddressBook>().remove(&address);
-    update_chart();
-    record(EventKind::AddressRemoved { id: address })
+fn remove_address(address: Principal) -> Result<(), String> {
+    if storage::get::<AddressBook>().is_controller(&address)
+        && storage::get::<AddressBook>().controllers().count() == 1
+    {
+        Err("The wallet must have at least one controller.".to_string())
+    } else {
+        storage::get_mut::<AddressBook>().remove(&address);
+        record(EventKind::AddressRemoved { id: address });
+        update_chart();
+        Ok(())
+    }
 }
 
 /***************************************************************************************************
@@ -573,7 +599,7 @@ struct GetEventsArgs {
 }
 
 /// Return the recent events observed by this canister.
-#[query(guard = "is_custodian")]
+#[query(guard = "is_custodian_or_controller")]
 fn get_events(args: Option<GetEventsArgs>) -> &'static [Event] {
     if let Some(GetEventsArgs { from, to }) = args {
         events::get_events(from, to)
@@ -597,7 +623,7 @@ struct GetChartArgs {
     precision: Option<u64>,
 }
 
-#[query(guard = "is_custodian")]
+#[query(guard = "is_custodian_or_controller")]
 fn get_chart(args: Option<GetChartArgs>) -> Vec<(u64, u64)> {
     let chart = storage::get_mut::<Vec<ChartTick>>();
 
@@ -648,11 +674,11 @@ fn is_controller() -> Result<(), String> {
 }
 
 /// Check if the caller is a custodian.
-fn is_custodian() -> Result<(), String> {
+fn is_custodian_or_controller() -> Result<(), String> {
     let caller = &caller();
-    if storage::get::<AddressBook>().is_custodian(caller) || &api::id() == caller {
+    if storage::get::<AddressBook>().is_controller_or_custodian(caller) || &api::id() == caller {
         Ok(())
     } else {
-        Err("Only a custodian can call this method.".to_string())
+        Err("Only a controller or custodian can call this method.".to_string())
     }
 }