diff --git a/cmd/mesh-controller/new_module_on_merge_test.go b/cmd/mesh-controller/new_module_on_merge_test.go
index a022c869..9dc955ea 100644
--- a/cmd/mesh-controller/new_module_on_merge_test.go
+++ b/cmd/mesh-controller/new_module_on_merge_test.go
@@ -4,6 +4,7 @@ package main
 
 import (
 	"context"
+	"sync"
 	"testing"
 
 	"git.novox.be/novox/mesh-sdk/go/catalogue"
@@ -11,12 +12,16 @@ import (
 	"github.com/novox/mesh-controller/internal/link"
 )
 
-// asksWithPaths records every build a merge asks for, as repository, path and ref.
+// asksWithPaths records every build a merge asks for, as repository, path and ref. The merge window's goroutine and
+// the test's own may ask at once, so the record is written under a lock (issue 1543).
 func asksWithPaths(t *testing.T) *[][3]string {
 	t.Helper()
 	var asked [][3]string
+	var mu sync.Mutex
 	was := askABuild
 	askABuild = func(_ context.Context, source buildSource, path, ref string) (string, error) {
+		mu.Lock()
+		defer mu.Unlock()
 		asked = append(asked, [3]string{source.Repository, path, ref})
 		return "b-" + path, nil
 	}