diff --git a/src/main.rs b/src/main.rs
index f7b896c..e467b2f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -502,6 +502,7 @@ type Matches<'src> = HashMap<Symbol, &'src Sexp>;
 
 // DONE?: there. can. be. at most. one. match.
 // i'm happy that this is faster, but it might be worth going back to the per-variable thing
+#[must_use]
 pub fn matches<'src>(vars: &[Symbol], lhs: &Sexp, expr: &'src Sexp) -> Option<Matches<'src>> {
     match (lhs, expr) {
         (Sexp::Atom(a), Sexp::Atom(b)) => {
@@ -526,13 +527,14 @@ pub fn matches<'src>(vars: &[Symbol], lhs: &Sexp, expr: &'src Sexp) -> Option<Ma
         }
         (Sexp::List(_), Sexp::Atom(_)) => None,
         (Sexp::List(xs), Sexp::List(ys)) => {
-            if xs.len() == ys.len() {
+            if xs == ys {
+                Some(HashMap::with_capacity(0))
+            } else if xs.len() == ys.len() {
                 xs.iter()
                     .zip(ys)
                     .map(|(lhs, expr)| matches(vars, lhs, expr))
                     .reduce(|a, b| match (a, b) {
-                        (None, _) => None,
-                        (_, None) => None,
+                        (None, _) | (_, None) => None,
                         (Some(a), Some(b)) => merge_matches(vars, a, b),
                     })
                     .unwrap_or(None)