diff --git a/modules/mesh-delivery-checker/internal/check/failure_words_test.go b/modules/mesh-delivery-checker/internal/check/failure_words_test.go
index 9334b9d..ef95b47 100644
--- a/modules/mesh-delivery-checker/internal/check/failure_words_test.go
+++ b/modules/mesh-delivery-checker/internal/check/failure_words_test.go
@@ -1,6 +1,7 @@
 package check
 
 import (
+	"encoding/json"
 	"strings"
 	"testing"
 
@@ -54,3 +55,19 @@ func TestWordsLongerThanTheBoundKeepTheirLastLines(t *testing.T) { // 1901/AC1
 	}
 	_ = delivery.VerdictFail
 }
+
+// An older build module sends no "cause": its answer is still accepted and the verdict's words are its output, as before.
+func TestAnOlderBuildModulesFailureIsAcceptedWithItsWholeOutput(t *testing.T) { // 1901/AC3
+	old := `{"package":"p","test":"TestX","output":"x_test.go:1: logged\nx_test.go:2: the old kept lines"}`
+	var f Failure
+	if err := json.Unmarshal([]byte(old), &f); err != nil || f.Cause != "" {
+		t.Fatalf("1901/AC3: err = %v, cause = %q", err, f.Cause)
+	}
+	v := levelFailure(&target{}, "unit", Tested{Failed: []Failure{f}})
+	if v.Failed.Words != "x_test.go:1: logged\nx_test.go:2: the old kept lines" {
+		t.Errorf("1901/AC3: words = %q", v.Failed.Words)
+	}
+	if got := firstFailed(*v); !strings.Contains(got, "logged") || strings.Contains(got, "\n") {
+		t.Errorf("1901/AC3: status = %q", got)
+	}
+}