diff --git a/language/parse.go b/language/parse.go
index 053336e2..1bac5b10 100644
--- a/language/parse.go
+++ b/language/parse.go
@@ -166,7 +166,12 @@ func ParseAcceptLanguage(s string) (tag []Tag, q []float32, err error) {
 		}
 	}()
 
-	if strings.Count(s, "-") > 1000 {
+	// Reject excessively large input to avoid quadratic behavior in the
+	// scanner (see Go issue 56152). Underscores are treated as separators
+	// equivalent to dashes (they are normalized to '-' during scanning), so
+	// they must be counted here as well; otherwise an underscore-separated
+	// value bypasses this limit.
+	if strings.Count(s, "-")+strings.Count(s, "_") > 1000 {
 		return nil, nil, errTagListTooLarge
 	}
 
diff --git a/language/parse_test.go b/language/parse_test.go
index 0eee033e..116a1edc 100644
--- a/language/parse_test.go
+++ b/language/parse_test.go
@@ -406,4 +406,11 @@ func TestParseAcceptLanguageTooBig(t *testing.T) {
 	if err != errTagListTooLarge {
 		t.Errorf("ParseAcceptLanguage() unexpected error: got %v, want %v", err, errTagListTooLarge)
 	}
+
+	// Underscores are normalized to dashes by the scanner, so they are
+	// separators too and must be subject to the same limit.
+	u := strings.Repeat("en_x_a_", 333) + "en_x_a"
+	if _, _, err = ParseAcceptLanguage(u); err != errTagListTooLarge {
+		t.Errorf("ParseAcceptLanguage(underscores) unexpected error: got %v, want %v", err, errTagListTooLarge)
+	}
 }