diff --git a/csexp/sexp.go b/csexp/sexp.go
index 6f696e9..9876293 100644
--- a/csexp/sexp.go
+++ b/csexp/sexp.go
@@ -147,7 +147,12 @@ func parseList(input gigaparsec.State[byte]) (gigaparsec.Result[byte, Sexp], err
 }
 
 func Parse(data []byte) (Sexp, error) {
-	result, err := parseSexp(gigaparsec.MakeState(cursor.NewSlice(data)))
+	parser := gigaparsec.Seq2(
+		parseSexp,
+		gigaparsec.End[byte](),
+		func(s Sexp, _ struct{}) Sexp { return s },
+	)
+	result, err := parser(gigaparsec.MakeState(cursor.NewSlice(data)))
 	if err != nil {
 		return nil, fmt.Errorf("csexp.Parse: %w", err)
 	}
diff --git a/csexp/sexp_test.go b/csexp/sexp_test.go
index a8126b7..851225e 100644
--- a/csexp/sexp_test.go
+++ b/csexp/sexp_test.go
@@ -6,6 +6,7 @@ import (
 
 	"git.codemonkeysoftware.net/b/peachy-go/csexp"
 	"git.codemonkeysoftware.net/b/peachy-go/csexp/gen"
+	"github.com/shoenig/test"
 	"github.com/shoenig/test/must"
 	"pgregory.net/rapid"
 )
@@ -33,6 +34,19 @@ func TestStringAndParseEqual(t *testing.T) {
 	})
 }
 
+func TestParseFails(t *testing.T) {
+	t.Run("with extra input", rapid.MakeCheck(func(t *rapid.T) {
+		sexp := gen.Sexp().Draw(t, "sexp")
+		b := []byte(sexp.String())
+		extra := rapid.SliceOfN(rapid.Byte(), 1, -1).Draw(t, "extra")
+		b = append(b, extra...)
+		parsed, err := csexp.Parse(b)
+		test.Error(t, err)
+		test.Nil(t, parsed)
+	}))
+	t.Run("with too little input", rapid.MakeCheck(func(t *rapid.T) {}))
+}
+
 func TestCloneEqual(t *testing.T) {
 	rapid.Check(t, func(t *rapid.T) {
 		sexp := gen.Sexp().Draw(t, "sexp")