diff --git a/jackrabbit-jcr-commons/src/main/java/org/apache/jackrabbit/commons/NamespaceHelper.java b/jackrabbit-jcr-commons/src/main/java/org/apache/jackrabbit/commons/NamespaceHelper.java
index 16851cd57cc..978269a5eb0 100644
--- a/jackrabbit-jcr-commons/src/main/java/org/apache/jackrabbit/commons/NamespaceHelper.java
+++ b/jackrabbit-jcr-commons/src/main/java/org/apache/jackrabbit/commons/NamespaceHelper.java
@@ -53,6 +53,11 @@ public class NamespaceHelper {
      */
     private final Session session;
 
+    /**
+     * Current namespace registry.
+     */
+    private NamespaceRegistry namespaceRegistry;
+
     /**
      * Creates a namespace helper for the given session.
      *
@@ -60,6 +65,19 @@ public class NamespaceHelper {
      */
     public NamespaceHelper(Session session) {
         this.session = session;
+        // will be set on lazily
+        this.namespaceRegistry = null;
+    }
+
+    /**
+        Get the namespace registry; needs to be done on-demand because the constructor
+        does not allow RepositoryException
+     */
+    private NamespaceRegistry getNamespaceRegistry() throws RepositoryException {
+        if (namespaceRegistry == null) {
+            namespaceRegistry = session.getWorkspace().getNamespaceRegistry();
+        }
+        return namespaceRegistry;
     }
 
     /**
@@ -197,8 +215,7 @@ public String getJcrName(String name)
      */
     public String registerNamespace(String prefix, String uri)
             throws RepositoryException {
-        NamespaceRegistry registry =
-            session.getWorkspace().getNamespaceRegistry();
+        NamespaceRegistry registry = getNamespaceRegistry();
         try {
             // Check if the namespace is registered
             registry.getPrefix(uri);
diff --git a/jackrabbit-jcr-commons/src/test/java/org/apache/jackrabbit/commons/NamespaceHelperTest.java b/jackrabbit-jcr-commons/src/test/java/org/apache/jackrabbit/commons/NamespaceHelperTest.java
index 17c849d1344..f4baa5c62e7 100644
--- a/jackrabbit-jcr-commons/src/test/java/org/apache/jackrabbit/commons/NamespaceHelperTest.java
+++ b/jackrabbit-jcr-commons/src/test/java/org/apache/jackrabbit/commons/NamespaceHelperTest.java
@@ -233,5 +233,8 @@ public void testRegisterNamespace() throws RepositoryException {
         assertEquals("test1", nsReg.getPrefix("test1:"));
         assertEquals("test2", nsReg.getPrefix("test2"));
         assertEquals("ns6", nsReg.getPrefix("test3:"));
+
+        // check invocation count for getNamespaceRegistry (JCR-5161)
+        Mockito.verify(workspace, Mockito.times(1)).getNamespaceRegistry();
     }
 }