diff --git a/pkg/parse/parse.go b/pkg/parse/parse.go
index ca92b62..4b398c6 100644
--- a/pkg/parse/parse.go
+++ b/pkg/parse/parse.go
@@ -445,7 +445,7 @@ func (s *InputSchema) ReadLine(l *line) error {
 	} else {
 		pushError(fmt.Errorf("no message key %q in incoming log", s.MessageKey))
 	}
-	if lvl, ok := l.fields[s.LevelKey]; ok {
+	if lvl, ok := l.fields[s.LevelKey]; s.LevelFormat != nil && ok {
 		if parsed, err := s.LevelFormat(lvl); err != nil {
 			pushError(fmt.Errorf("level key %q: %w", s.LevelKey, err))
 		} else {
diff --git a/pkg/parse/parse_fuzz_test.go b/pkg/parse/parse_fuzz_test.go
new file mode 100644
index 0000000..5d8d50e
--- /dev/null
+++ b/pkg/parse/parse_fuzz_test.go
@@ -0,0 +1,50 @@
+//go:build go1.18
+// +build go1.18
+
+package parse
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/logrusorgru/aurora/v3"
+)
+
+func FuzzReadLog(f *testing.F) {
+	f.Add([]byte(`{}`))
+	f.Add([]byte(`{"ts": 1234, "level": "info", "msg": "hello"}\n`))
+	f.Add([]byte(`{"ts": 1234, "level": "info", "msg": "hello"}\n` +
+		`{"ts": 1235, "level": "warn", "msg": "line 2"}`))
+	f.Add([]byte(`{"ts": 1234, "level": "info", "msg": "hello"}\n{}\n`))
+	jq, err := CompileJQ(".")
+	if err != nil {
+		f.Fatalf("jq: %v", err)
+	}
+
+	f.Fuzz(func(t *testing.T, in []byte) {
+		inbuf := bytes.NewReader(in)
+		ins := &InputSchema{
+			Strict: false,
+		}
+		errbuf := new(bytes.Buffer)
+		outs := &OutputSchema{
+			Formatter: &DefaultOutputFormatter{
+				Aurora: aurora.NewAurora(true),
+			},
+			EmitErrorFn: func(msg string) {
+				errbuf.WriteString(msg)
+				errbuf.WriteString("\n")
+			},
+		}
+		outbuf := new(bytes.Buffer)
+		if _, err := ReadLog(inbuf, outbuf, ins, outs, jq); err != nil {
+			t.Fatal(err)
+		}
+		if got, want := bytes.Count(outbuf.Bytes(), []byte("\n")), bytes.Count(in, []byte("\n")); got < want {
+			t.Errorf("output: line count:\n  got:   %v\n want: >=%v", got, want)
+		}
+		if errbuf.Len() > 0 {
+			t.Logf("errors: %v", errbuf.String())
+		}
+	})
+}
diff --git a/pkg/parse/testdata/fuzz/FuzzReadLog/13d14f3a75b99f154b8bc6a3e2032b22446430e5a905f99d0d9cb673c3feda15 b/pkg/parse/testdata/fuzz/FuzzReadLog/13d14f3a75b99f154b8bc6a3e2032b22446430e5a905f99d0d9cb673c3feda15
new file mode 100644
index 0000000..8c5fa7a
--- /dev/null
+++ b/pkg/parse/testdata/fuzz/FuzzReadLog/13d14f3a75b99f154b8bc6a3e2032b22446430e5a905f99d0d9cb673c3feda15
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("{\"\":0}")