diff --git a/external-sop/src/main/java/sop/external/operation/DetachedVerifyExternal.java b/external-sop/src/main/java/sop/external/operation/DetachedVerifyExternal.java
index 05318d4..b9cd182 100644
--- a/external-sop/src/main/java/sop/external/operation/DetachedVerifyExternal.java
+++ b/external-sop/src/main/java/sop/external/operation/DetachedVerifyExternal.java
@@ -31,7 +31,7 @@ public class DetachedVerifyExternal implements DetachedVerify {
     private final List<String> commandList = new ArrayList<>();
     private final List<String> envList;
 
-    private Set<InputStream> certs = new HashSet<>();
+    private final Set<InputStream> certs = new HashSet<>();
     private InputStream signatures;
     private int certCounter = 0;