diff --git a/frame/fast-unstake/src/lib.rs b/frame/fast-unstake/src/lib.rs
index 7f226826cbc53..f2faeebc13478 100644
--- a/frame/fast-unstake/src/lib.rs
+++ b/frame/fast-unstake/src/lib.rs
@@ -91,7 +91,7 @@ pub mod pallet {
 		DispatchResult,
 	};
 	use sp_staking::{EraIndex, StakingInterface};
-	use sp_std::{prelude::*, vec::Vec};
+	use sp_std::{collections::btree_set::BTreeSet, prelude::*, vec::Vec};
 	pub use weights::WeightInfo;
 
 	#[derive(scale_info::TypeInfo, codec::Encode, codec::Decode, codec::MaxEncodedLen)]
@@ -429,9 +429,9 @@ pub mod pallet {
 				}
 			};
 
-			let check_stash = |stash, deposit, eras_checked: &mut u32| {
+			let check_stash = |stash, deposit, eras_checked: &mut BTreeSet<EraIndex>| {
 				let is_exposed = unchecked_eras_to_check.iter().any(|e| {
-					eras_checked.saturating_inc();
+					eras_checked.insert(*e);
 					T::Staking::is_exposed_in_era(&stash, e)
 				});
 
@@ -452,7 +452,7 @@ pub mod pallet {
 				<T as Config>::WeightInfo::on_idle_unstake()
 			} else {
 				// eras checked so far.
-				let mut eras_checked = 0u32;
+				let mut eras_checked = BTreeSet::<EraIndex>::new();
 
 				let pre_length = stashes.len();
 				let stashes: BoundedVec<(T::AccountId, BalanceOf<T>), T::BatchSize> = stashes
@@ -468,7 +468,7 @@ pub mod pallet {
 				log!(
 					debug,
 					"checked {:?} eras, pre stashes: {:?}, post: {:?}",
-					eras_checked,
+					eras_checked.len(),
 					pre_length,
 					post_length,
 				);
@@ -489,7 +489,9 @@ pub mod pallet {
 					},
 				}
 
-				<T as Config>::WeightInfo::on_idle_check(validator_count * eras_checked)
+				<T as Config>::WeightInfo::on_idle_check(
+					validator_count * eras_checked.len() as u32,
+				)
 			}
 		}
 	}
diff --git a/frame/staking/src/pallet/impls.rs b/frame/staking/src/pallet/impls.rs
index a7190d70c7061..db9aeba6fb58e 100644
--- a/frame/staking/src/pallet/impls.rs
+++ b/frame/staking/src/pallet/impls.rs
@@ -1569,7 +1569,7 @@ impl<T: Config> StakingInterface for Pallet<T> {
 	}
 
 	fn force_unstake(who: Self::AccountId) -> sp_runtime::DispatchResult {
-		let num_slashing_spans = Self::slashing_spans(&who).iter().count() as u32;
+		let num_slashing_spans = Self::slashing_spans(&who).map_or(0, |s| s.iter().count() as u32);
 		Self::force_unstake(RawOrigin::Root.into(), who.clone(), num_slashing_spans)
 	}
 
diff --git a/frame/staking/src/pallet/mod.rs b/frame/staking/src/pallet/mod.rs
index 1d5babe7ffa8f..f04d13d05f468 100644
--- a/frame/staking/src/pallet/mod.rs
+++ b/frame/staking/src/pallet/mod.rs
@@ -982,7 +982,8 @@ pub mod pallet {
 			// `BondingDuration` to proceed with the unbonding.
 			let maybe_withdraw_weight = {
 				if unlocking == T::MaxUnlockingChunks::get() as usize {
-					let real_num_slashing_spans = Self::slashing_spans(&controller).iter().count();
+					let real_num_slashing_spans =
+						Self::slashing_spans(&controller).map_or(0, |s| s.iter().count());
 					Some(Self::do_withdraw_unbonded(&controller, real_num_slashing_spans as u32)?)
 				} else {
 					None
diff --git a/frame/staking/src/tests.rs b/frame/staking/src/tests.rs
index fc6fc68e66d5d..5c2766deee307 100644
--- a/frame/staking/src/tests.rs
+++ b/frame/staking/src/tests.rs
@@ -5725,3 +5725,54 @@ fn scale_validator_count_errors() {
 		);
 	})
 }
+
+mod staking_interface {
+	use frame_support::storage::with_storage_layer;
+	use sp_staking::StakingInterface;
+
+	use super::*;
+
+	#[test]
+	fn force_unstake_with_slash_works() {
+		ExtBuilder::default().build_and_execute(|| {
+			// without slash
+			let _ = with_storage_layer::<(), _, _>(|| {
+				// bond an account, can unstake
+				assert_eq!(Staking::bonded(&11), Some(10));
+				assert_ok!(<Staking as StakingInterface>::force_unstake(11));
+				Err(DispatchError::from("revert"))
+			});
+
+			// bond again and add a slash, still can unstake.
+			assert_eq!(Staking::bonded(&11), Some(10));
+			add_slash(&11);
+			assert_ok!(<Staking as StakingInterface>::force_unstake(11));
+		});
+	}
+
+	#[test]
+	fn do_withdraw_unbonded_with_wrong_slash_spans_works_as_expected() {
+		ExtBuilder::default().build_and_execute(|| {
+			on_offence_now(
+				&[OffenceDetails {
+					offender: (11, Staking::eras_stakers(active_era(), 11)),
+					reporters: vec![],
+				}],
+				&[Perbill::from_percent(100)],
+			);
+
+			assert_eq!(Staking::bonded(&11), Some(10));
+
+			assert_noop!(
+				Staking::withdraw_unbonded(RuntimeOrigin::signed(10), 0),
+				Error::<Test>::IncorrectSlashingSpans
+			);
+
+			let num_slashing_spans = Staking::slashing_spans(&11).map_or(0, |s| s.iter().count());
+			assert_ok!(Staking::withdraw_unbonded(
+				RuntimeOrigin::signed(10),
+				num_slashing_spans as u32
+			));
+		});
+	}
+}