diff --git a/src/algorithms/kk.rs b/src/algorithms/kk.rs
index 944fe701..60b970fc 100644
--- a/src/algorithms/kk.rs
+++ b/src/algorithms/kk.rs
@@ -1,3 +1,4 @@
+use super::Error;
 use std::collections::BinaryHeap;
 use std::ops::Sub;
 use std::ops::SubAssign;
@@ -9,7 +10,7 @@ use num::Zero;
 /// # Differences with the k-partitioning implementation
 ///
 /// This function has better performance than [kk] called with `num_parts == 2`.
-fn kk_bipart<T>(partition: &mut [usize], weights: impl IntoIterator<Item = T>)
+fn kk_bipart<T>(partition: &mut [usize], weights: impl Iterator<Item = T>)
 where
     T: Ord + Sub<Output = T>,
 {
@@ -17,10 +18,6 @@ where
         .into_iter()
         .zip(0..) // Keep track of the weights' indicies
         .collect();
-    assert_eq!(partition.len(), weights.len());
-    if weights.is_empty() {
-        return;
-    }
 
     // Core algorithm: find the imbalance of the partition.
     // "opposites" is built in this loop to backtrack the solution. It tracks weights that must end
@@ -54,30 +51,15 @@ where
 fn kk<T, I>(partition: &mut [usize], weights: I, num_parts: usize)
 where
     T: Zero + Ord + Sub<Output = T> + SubAssign + Copy,
-    I: IntoIterator<Item = T>,
-    <I as IntoIterator>::IntoIter: ExactSizeIterator,
+    I: Iterator<Item = T> + ExactSizeIterator,
 {
-    let weights = weights.into_iter();
-    let num_weights = weights.len();
-    assert_eq!(partition.len(), num_weights);
-    if num_weights == 0 {
-        return;
-    }
-    if num_parts < 2 {
-        return;
-    }
-    if num_parts == 2 {
-        // The bi-partitioning is a special case that can be handled faster than
-        // the general case.
-        return kk_bipart(partition, weights);
-    }
-
     // Initialize "m", a "k*num_weights" matrix whose first column is "weights".
+    let weight_count = weights.len();
     let mut m: BinaryHeap<Vec<(T, usize)>> = weights
         .zip(0..)
         .map(|(w, id)| {
             let mut v: Vec<(T, usize)> = (0..num_parts)
-                .map(|p| (T::zero(), num_weights * p + id))
+                .map(|p| (T::zero(), weight_count * p + id))
                 .collect();
             v[0].0 = w;
             v
@@ -88,7 +70,7 @@ where
     // largest weights in two different parts, the largest weight of each row is put into the same
     // part as the smallest one, and so on.
 
-    let mut opposites = Vec::with_capacity(num_weights);
+    let mut opposites = Vec::with_capacity(weight_count);
     while 2 <= m.len() {
         let a = m.pop().unwrap();
         let b = m.pop().unwrap();
@@ -119,7 +101,7 @@ where
     // Backtracking. Same as the bi-partitioning case.
 
     // parts = [ [m0i] for m0i in m[0] ]
-    let mut parts: Vec<usize> = vec![0; num_parts * num_weights];
+    let mut parts: Vec<usize> = vec![0; num_parts * weight_count];
     let imbalance = m.pop().unwrap(); // first and last element of "m".
     for (i, w) in imbalance.into_iter().enumerate() {
         // Put each remaining element in a different part.
@@ -160,14 +142,30 @@ where
     W::Item: Zero + Ord + Sub<Output = W::Item> + SubAssign + Copy,
 {
     type Metadata = ();
-    type Error = std::convert::Infallible;
+    type Error = Error;
 
     fn partition(
         &mut self,
         part_ids: &mut [usize],
         weights: W,
     ) -> Result<Self::Metadata, Self::Error> {
-        kk(part_ids, weights, self.part_count);
+        if self.part_count < 2 || part_ids.len() < 2 {
+            return Ok(());
+        }
+        let weights = weights.into_iter();
+        if weights.len() != part_ids.len() {
+            return Err(Error::InputLenMismatch {
+                expected: part_ids.len(),
+                actual: weights.len(),
+            });
+        }
+        if self.part_count == 2 {
+            // The bi-partitioning is a special case that can be handled faster
+            // than the general case.
+            kk_bipart(part_ids, weights);
+        } else {
+            kk(part_ids, weights, self.part_count);
+        }
         Ok(())
     }
 }