diff --git a/src/lib.rs b/src/lib.rs
index df22809..b509465 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -52,8 +52,11 @@ impl Segmenter {
     }
 
     /// Appends list of words that is the best segmentation of `text` to `out`
-    pub fn segment(&self, text: &str, out: &mut Vec<String>) {
-        SegmentState::new(&Ascii::new(text), &self, out).run()
+    ///
+    /// Requires that the input `text` consists of lowercase ASCII characters only. Otherwise,
+    /// returns `Err(InvalidCharacter)`.
+    pub fn segment(&self, text: &str, out: &mut Vec<String>) -> Result<(), InvalidCharacter> {
+        Ok(SegmentState::new(Ascii::new(text)?, &self, out).run())
     }
 
     fn score(&self, word: &str, previous: Option<&str>) -> f64 {
@@ -90,7 +93,7 @@ impl Segmenter {
 
 struct SegmentState<'a> {
     data: &'a Segmenter,
-    text: &'a Ascii,
+    text: Ascii<'a>,
     memo: HashMap<MemoKey, (f64, Range<usize>)>,
     split_cache: Vec<usize>,
     result: &'a mut Vec<String>,
@@ -98,7 +101,7 @@ struct SegmentState<'a> {
 }
 
 impl<'a> SegmentState<'a> {
-    fn new(text: &'a Ascii, data: &'a Segmenter, result: &'a mut Vec<String>) -> Self {
+    fn new(text: Ascii<'a>, data: &'a Segmenter, result: &'a mut Vec<String>) -> Self {
         Self {
             data,
             text,
@@ -172,19 +175,16 @@ impl<'a> SegmentState<'a> {
 
 type MemoKey = (Range<usize>, Range<usize>);
 
-struct Ascii(Vec<u8>);
+#[derive(Debug)]
+struct Ascii<'a>(&'a [u8]);
 
-impl Ascii {
-    fn new(s: &str) -> Self {
-        Self(
-            s.chars()
-                .filter_map(|c| match c.is_ascii_alphanumeric() {
-                    true => Some(c.to_ascii_lowercase()),
-                    false => None,
-                })
-                .collect::<std::string::String>()
-                .into_bytes(),
-        )
+impl<'a> Ascii<'a> {
+    fn new(s: &'a str) -> Result<Self, InvalidCharacter> {
+        let bytes = s.as_bytes();
+        match bytes.iter().all(|b| b.is_ascii_lowercase()) {
+            true => Ok(Self(bytes)),
+            false => Err(InvalidCharacter),
+        }
     }
 
     fn len(&self) -> usize {
@@ -192,7 +192,7 @@ impl Ascii {
     }
 }
 
-impl Index<Range<usize>> for Ascii {
+impl<'a> Index<Range<usize>> for Ascii<'a> {
     type Output = str;
 
     fn index(&self, index: Range<usize>) -> &Self::Output {
@@ -202,6 +202,17 @@ impl Index<Range<usize>> for Ascii {
     }
 }
 
+#[derive(Debug)]
+pub struct InvalidCharacter;
+
+impl std::error::Error for InvalidCharacter {}
+
+impl std::fmt::Display for InvalidCharacter {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("invalid character")
+    }
+}
+
 type HashMap<K, V> = std::collections::HashMap<K, V, ahash::RandomState>;
 
 const DEFAULT_LIMIT: usize = 24;
@@ -211,7 +222,8 @@ const SEGMENT_SIZE: usize = 250;
 pub mod tests {
     #[test]
     fn test_clean() {
-        let text = super::Ascii::new("Can't buy me love!");
+        super::Ascii::new("Can't buy me love!").unwrap_err();
+        let text = super::Ascii::new("cantbuymelove").unwrap();
         assert_eq!(&text[0..text.len()], "cantbuymelove");
     }
 }
diff --git a/src/test_cases.rs b/src/test_cases.rs
index 0f53c99..8d86eb3 100644
--- a/src/test_cases.rs
+++ b/src/test_cases.rs
@@ -10,15 +10,17 @@ pub fn run(segmenter: &Segmenter) {
 
 pub fn assert_segments(segmenter: &Segmenter, s: &[&str]) {
     let mut out = Vec::new();
-    segmenter.segment(&s.join(""), &mut out);
+    segmenter.segment(&s.join(""), &mut out).unwrap();
     let cmp = out.iter().map(|s| &*s).collect::<Vec<_>>();
     assert_eq!(cmp, s);
 }
 
 pub fn check_segments(segmenter: &Segmenter, s: &[&str]) -> bool {
     let mut out = Vec::new();
-    segmenter.segment(&s.join(""), &mut out);
-    s == out.iter().map(|s| &*s).collect::<Vec<_>>()
+    match segmenter.segment(&s.join(""), &mut out) {
+        Ok(()) => s == out.iter().map(|s| &*s).collect::<Vec<_>>(),
+        Err(_) => false,
+    }
 }
 
 /// Built-in test cases