log registration of invalid namespace names

diff --git a/oak-jcr/src/test/java/org/apache/jackrabbit/oak/jcr/RepositoryTest.java b/oak-jcr/src/test/java/org/apache/jackrabbit/oak/jcr/RepositoryTest.java
index f113e4e0d2..7178a36f7f 100644
--- a/oak-jcr/src/test/java/org/apache/jackrabbit/oak/jcr/RepositoryTest.java
+++ b/oak-jcr/src/test/java/org/apache/jackrabbit/oak/jcr/RepositoryTest.java
@@ -1996,6 +1996,24 @@ public class RepositoryTest extends AbstractRepositoryTest {
         assertFalse(asList(nsReg.getURIs()).contains("file:///foo"));
     }

+    @Test
+    public void testNamespaceNames() throws RepositoryException {
+        NamespaceRegistry nsReg =
+                getAdminSession().getWorkspace().getNamespaceRegistry();
+
+        // valid
+        nsReg.registerNamespace("foo", "https://example.com");
+        nsReg.unregisterNamespace("foo");
+
+        // invalid
+        try {
+            nsReg.registerNamespace("foo", "example.com");
+            fail("should not register invalid namespace name");
+        } finally {
+            nsReg.unregisterNamespace("foo");
+        }
+    }