diff --git a/Cargo.toml b/Cargo.toml
index 9e5112a..ab9cf5b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "sketchlib"
-version = "0.4.1"
+version = "0.4.2"
 authors = [
     "John Lees <jlees@ebi.ac.uk>",
     "Nicholas Croucher <n.croucher@imperial.ac.uk>",
diff --git a/src/hashing/aahash_iterator.rs b/src/hashing/aahash_iterator.rs
index 5f87413..408a4a4 100644
--- a/src/hashing/aahash_iterator.rs
+++ b/src/hashing/aahash_iterator.rs
@@ -30,16 +30,15 @@ pub struct AaHashIterator {
 }
 
 impl RollHash for AaHashIterator {
-    fn set_k(&mut self, k: usize) {
+    fn set_k(&mut self, k: usize) -> anyhow::Result<()> {
         self.k = k;
         if let Some(new_it) = Self::new_iterator(0, &self.level, &self.seq, k) {
             self.fh = new_it.0;
             self.index = new_it.1;
+            Ok(())
         } else {
-            panic!(
-                "K-mer larger than smallest valid sequence, which is:\n{}",
-                std::str::from_utf8(&self.seq).unwrap()
-            );
+            let seq_str = std::str::from_utf8(&self.seq).unwrap_or("<not a valid UTF-8 string>");
+            Err(anyhow::anyhow!("K-mer size {} larger than smallest valid sequence {}", k, seq_str))
         }
     }
 
diff --git a/src/hashing/mod.rs b/src/hashing/mod.rs
index e4cb14a..04a2359 100644
--- a/src/hashing/mod.rs
+++ b/src/hashing/mod.rs
@@ -114,7 +114,7 @@ fn swapbits3263(v: u64) -> u64 {
 /// Rolling functions supported by both ntHash and aaHash
 pub trait RollHash: Iterator<Item = u64> {
     /// Set the k-mer size
-    fn set_k(&mut self, k: usize);
+    fn set_k(&mut self, k: usize) -> anyhow::Result<()>;
     /// Get the current hash
     fn curr_hash(&self) -> u64;
     /// The type of sequence being hashed
diff --git a/src/hashing/nthash_iterator.rs b/src/hashing/nthash_iterator.rs
index adf48ea..da0664e 100644
--- a/src/hashing/nthash_iterator.rs
+++ b/src/hashing/nthash_iterator.rs
@@ -46,14 +46,16 @@ pub struct NtHashIterator {
 }
 
 impl RollHash for NtHashIterator {
-    fn set_k(&mut self, k: usize) {
+    fn set_k(&mut self, k: usize) -> anyhow::Result<()> {
         if k != self.k {
             self.k = k;
             self.offset_idx = 0; // rewind: offsets must be re-traversed for each k
             if self.next_iterator(0).is_none() {
-                panic!("K-mer larger than smallest valid sequence");
+                let seq_str = std::str::from_utf8(&self.seq).unwrap_or("<not a valid UTF-8 string>");
+                return Err(anyhow::anyhow!("K-mer size {} larger than smallest valid sequence {}", k, seq_str))
             }
         }
+        Ok(())
     }
 
     /// Retrieve the current hash (minimum of forward and reverse complement hashes)
@@ -95,7 +97,7 @@ impl NtHashIterator {
         rc: bool,
         min_qual: u8,
         reads: bool,
-    ) -> Vec<Self> {
+    ) -> anyhow::Result<Vec<Self>> {
         let mut seq = Vec::new();
         let mut offsets = Vec::new();
         let mut acgt = [0, 0, 0, 0];
@@ -129,8 +131,8 @@ impl NtHashIterator {
             non_acgt,
             reads,
         };
-        hash_it.set_k(k);
-        vec![hash_it]
+        hash_it.set_k(k)?;
+        Ok(vec![hash_it])
     }
 
     #[cfg(target_arch = "wasm32")]
@@ -448,7 +450,7 @@ impl NtHashIterator {
             non_acgt,
             reads: false,
         };
-        hash_it.set_k(k);
+        hash_it.set_k(k).unwrap();
         hash_it
     }
 }
@@ -633,9 +635,9 @@ mod tests {
         // Simulate the sketch command: one iterator, multiple set_k calls in sequence.
         let mut it = NtHashIterator::from_seq(seq, 3, true);
         let hashes_k3: Vec<u64> = it.by_ref().collect();
-        it.set_k(5);
+        it.set_k(5).unwrap();
         let hashes_k5: Vec<u64> = it.by_ref().collect();
-        it.set_k(7);
+        it.set_k(7).unwrap();
         let hashes_k7: Vec<u64> = it.by_ref().collect();
         assert_eq!(hashes_k3, ref_hashes(seq, 3, true), "k=3 hashes wrong");
         assert_eq!(
diff --git a/src/inverted.rs b/src/inverted.rs
index 502e63d..0870cbb 100644
--- a/src/inverted.rs
+++ b/src/inverted.rs
@@ -379,7 +379,7 @@ impl Inverted {
                                 crate::io::NeedletailIterator::new(reader)
                             }).collect::<Vec<crate::io::NeedletailIterator>>();
 
-                                NtHashIterator::new(&mut records_readers, k, rc, min_qual, reads)
+                                NtHashIterator::new(&mut records_readers, k, rc, min_qual, reads).unwrap()
                                 .into_iter()
                                 .map(|it| Box::new(it) as Box<dyn RollHash>)
                                 .collect()
@@ -428,18 +428,18 @@ impl Inverted {
                 // Not yet written!
                 if !multientrysamples.contains(name) {
                     // Densifying now!
-                    Sketch::densify_bin(&mut sketch);
+                    Sketch::densify_bin(sketch.as_mut().unwrap());
                 } else {
                     // We'll need to densify afterwards, let's save the index
                     indexes.insert(genome_idx);
                 }
-                sketch_results[genome_idx] = sketch.iter().map(|h| *h as u16).collect();
+                sketch_results[genome_idx] = sketch.as_ref().unwrap().iter().map(|h| *h as u16).collect();
                 differentsamples.remove(name);
             } else {
                 // already written! We have to merge
                 for bin in 0..sketch_size {
                     let saved_sketch = &mut sketch_results[genome_idx][bin as usize];
-                    *saved_sketch = cmp::min(*saved_sketch, sketch[bin as usize] as u16);
+                    *saved_sketch = cmp::min(*saved_sketch, sketch.as_ref().unwrap()[bin as usize] as u16);
                 }
             }
         }
@@ -496,7 +496,7 @@ impl Inverted {
             };
 
             let (signs, densified) =
-                Sketch::get_signs(&mut **hash_it, k, &mut read_filter, sketch_size);
+                Sketch::get_signs(&mut **hash_it, k, &mut read_filter, sketch_size).unwrap();
             if densified {
                 logw("The query was densified", Some("trace"));
             }
diff --git a/src/sketch/mod.rs b/src/sketch/mod.rs
index e799a9e..573c0f2 100644
--- a/src/sketch/mod.rs
+++ b/src/sketch/mod.rs
@@ -171,7 +171,7 @@ impl Sketch {
         sketch_size: u64,
         rc: bool,
         min_count: u16,
-    ) -> Self {
+    ) -> anyhow::Result<Self> {
         let (_sketchsize64, num_bins, usigs_size) = num_bins(sketch_size);
         let flattened_size_u64 = usigs_size as usize * kmer_lengths.len();
         let mut usigs = aligned_sketch_vec_with_capacity(flattened_size_u64);
@@ -189,7 +189,7 @@ impl Sketch {
         let mut densified = false;
         for k in kmer_lengths {
             log::debug!("Running sketching at k={k}");
-            let (signs, k_densified) = Self::get_signs(seq_hashes, *k, &mut read_filter, num_bins);
+            let (signs, k_densified) = Self::get_signs(seq_hashes, *k, &mut read_filter, num_bins)?;
             densified |= k_densified;
             minhash_sum += (signs[0] as f64) / (u64::MAX as f64);
 
@@ -208,7 +208,7 @@ impl Sketch {
             seq_hashes.seq_len()
         };
 
-        Self {
+        Ok(Self {
             usigs,
             name: name.to_string(),
             index: None,
@@ -218,7 +218,7 @@ impl Sketch {
             densified,
             acgt,
             non_acgt,
-        }
+        })
     }
 
     /// Get the sketch bins for a sample, but do not transpose
@@ -227,13 +227,13 @@ impl Sketch {
         kmer_size: usize,
         filter: &mut Option<KmerFilter>,
         num_bins: u64,
-    ) -> (Vec<u64>, bool) {
+    ) -> anyhow::Result<(Vec<u64>, bool)> {
         // Setup storage for each k
         let mut signs = vec![u64::MAX; num_bins as usize];
         if let Some(read_filter) = filter {
             read_filter.clear();
         }
-        seq_hashes.set_k(kmer_size);
+        seq_hashes.set_k(kmer_size)?;
 
         // Calculate bin minima across all sequence
         for hash in seq_hashes.iter() {
@@ -241,7 +241,7 @@ impl Sketch {
         }
         // Densify
         let densified = Self::densify_bin(&mut signs);
-        (signs, densified)
+        Ok((signs, densified))
     }
 
     /// Get the sketch bins for a sample, but do not transpose
@@ -250,20 +250,20 @@ impl Sketch {
         kmer_size: usize,
         filter: &mut Option<KmerFilter>,
         num_bins: u64,
-    ) -> Vec<u64> {
+    ) -> anyhow::Result<Vec<u64>> {
         // Setup storage for each k
         let mut signs = vec![u64::MAX; num_bins as usize];
         if let Some(read_filter) = filter {
             read_filter.clear();
         }
-        seq_hashes.set_k(kmer_size);
+        seq_hashes.set_k(kmer_size)?;
 
         // Calculate bin minima across all sequence
         for hash in seq_hashes.iter() {
             Self::bin_sign(&mut signs, hash, num_bins, filter);
         }
 
-        signs
+        Ok(signs)
     }
 
     /// The name of the sample
@@ -467,7 +467,7 @@ impl fmt::Display for Sketch {
 ///     let reader = needletail::parse_fastx_file(fastx_path).unwrap();
 ///     let mut filtered_iters = vec![NeedletailFilterIterator::new(reader, want_ids)];
 ///
-///     sketch_data(&mut filtered_iters, opts)
+///     sketch_data(&mut filtered_iters, opts).unwrap()
 /// }
 ///
 /// let fastq_path_str = "tests/test_files_in/14412_3#82.contigs_velvet.fa.gz";
@@ -490,7 +490,7 @@ pub fn sketch_data<I: Iterator<Item = (Vec<u8>, Option<Vec<u8>>)>>(
     opts: SketchingOpts,
     #[cfg(feature = "3di")] convert_pdb: bool,
     #[cfg(feature = "3di")] struct_string: Option<String>,
-) -> Vec<Sketch> {
+) -> anyhow::Result<Vec<Sketch>> {
     // Read in sequence and set up rolling hash by alphabet type
     let mut hash_its: Vec<Box<dyn RollHash>> = match opts.seq_type {
         HashType::DNA => NtHashIterator::new(
@@ -499,7 +499,7 @@ pub fn sketch_data<I: Iterator<Item = (Vec<u8>, Option<Vec<u8>>)>>(
             opts.add_rc,
             opts.min_qual,
             opts.is_reads,
-        )
+        )?
         .into_iter()
         .map(|it| Box::new(it) as Box<dyn RollHash>)
         .collect(),
@@ -533,29 +533,28 @@ pub fn sketch_data<I: Iterator<Item = (Vec<u8>, Option<Vec<u8>>)>>(
         }
     };
 
-    hash_its
-        .iter_mut()
-        .enumerate()
-        .map(|(idx, hash_it)| {
-            let sample_name = if opts.concat_fasta {
-                format!("{}_{}", opts.name, idx + 1)
-            } else {
-                opts.name.to_string()
-            };
-            if hash_it.seq_len() == 0 {
-                panic!("{sample_name} has no valid sequence");
-            }
-            // Run the sketching
-            Sketch::new(
-                &mut **hash_it,
-                &sample_name,
-                &opts.k_vals,
-                opts.sketch_size,
-                opts.add_rc,
-                opts.min_count,
-            )
-        })
-        .collect::<Vec<Sketch>>()
+    let mut sketches: Vec<Sketch> = Vec::with_capacity(hash_its.len());
+    for(idx, hash_it) in hash_its.iter_mut().enumerate() {
+        let sample_name = if opts.concat_fasta {
+            format!("{}_{}", opts.name, idx + 1)
+        } else {
+            opts.name.to_string()
+        };
+        if hash_it.seq_len() == 0 {
+            return Err(anyhow::anyhow!("{sample_name} has no valid sequence"))
+        }
+        // Run the sketching
+        let sketch = Sketch::new(
+            &mut **hash_it,
+            &sample_name,
+            &opts.k_vals,
+            opts.sketch_size,
+            opts.add_rc,
+            opts.min_count,
+        )?;
+        sketches.push(sketch);
+    }
+    Ok(sketches)
 }
 
 #[cfg(not(target_arch = "wasm32"))]
@@ -656,7 +655,7 @@ pub fn sketch_files(
                         convert_pdb,
                         #[cfg(feature = "3di")]
                         di,
-                    )
+                    ).unwrap()
                 })
                 .for_each_with(tx, |tx, sketch| {
                     // Emit the sketch results to the writer thread