diff --git a/builder/builder-next/builder.go b/builder/builder-next/builder.go
index f2e111ff48..10c0083a31 100644
--- a/builder/builder-next/builder.go
+++ b/builder/builder-next/builder.go
@@ -241,7 +241,9 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.
 		}
 
 		defer func() {
+			b.mu.Lock()
 			delete(b.jobs, buildID)
+			b.mu.Unlock()
 		}()
 	}