diff --git a/lexer/src/lib.rs b/lexer/src/lib.rs
index b3d66ef..f7ad002 100644
--- a/lexer/src/lib.rs
+++ b/lexer/src/lib.rs
@@ -12,6 +12,7 @@ enum Resume { Here, AtNext }
 
 #[derive(Debug, Eq, PartialEq)]
 enum IterationResult {
+    Finish,
     Continue,
     Emit(Token, Resume),
     Error(String),
@@ -40,31 +41,39 @@ impl<T> Iterator for Lexer<T> where T: Iterator<Item=char> {
 
     fn next(&mut self) -> Option<Self::Item> {
         let mut buffer = String::new();
-        while let Some(peek) = self.input.peek().map(char::clone) {
+        loop {
+            let peek = self.input.peek().map(char::clone);
             let result = if buffer.is_empty() {
                 match peek {
-                    '(' => self.emit(Token::LeftParen, Resume::AtNext),
-                    ')' => self.emit(Token::RightParen, Resume::AtNext),
-                    c if c.is_whitespace() => IterationResult::Continue,
-                    c if c.is_alphabetic() => {
+                    Some('(') => self.emit(Token::LeftParen, Resume::AtNext),
+                    Some(')') => self.emit(Token::RightParen, Resume::AtNext),
+                    Some(c) if c.is_whitespace() => IterationResult::Continue,
+                    Some(c) if c.is_alphabetic() => {
                         buffer.push(c);
                         IterationResult::Continue
                     },
-                    c => self.fail(format!("Invalid character: {}", c)),
+                    Some(c) => self.fail(format!("Invalid character: {}", c)),
+                    // We found EOF and there's no pending string, so just finish.
+                    None => IterationResult::Finish,
                 }
             }
             else {
                 match peek {
-                    c if c.is_alphabetic() => {
+                    Some(c) if c.is_alphabetic() => {
                         buffer.push(c);
                         IterationResult::Continue
                     }
-                    c if c == '(' || c == ')' || c.is_whitespace() =>
+                    Some(c) if c == '(' || c == ')' || c.is_whitespace() =>
                         self.emit(Token::Id(buffer.clone()), Resume::Here),
-                    c => self.fail(format!("Invalid character: {}", c)),
+                    Some(c) => self.fail(format!("Invalid character: {}", c)),
+                    // Found EOF. Emit what we have and finish.
+                    // Note: the Resume argument doesn't matter in this case since the input
+                    // iterator will always be None from here on.
+                    None => self.emit(Token::Id(buffer.clone()), Resume::Here),
                 }
             };
             match result {
+                IterationResult::Finish => break,
                 IterationResult::Continue => self.input.next(),
                 IterationResult::Emit(token, resume) => {
                     if resume == Resume::AtNext {