diff --git a/src/main/java/org/apache/commons/text/lookup/DnsStringLookup.java b/src/main/java/org/apache/commons/text/lookup/DnsStringLookup.java
index 028879c27f..bbc38984de 100644
--- a/src/main/java/org/apache/commons/text/lookup/DnsStringLookup.java
+++ b/src/main/java/org/apache/commons/text/lookup/DnsStringLookup.java
@@ -87,6 +87,9 @@ public String lookup(final String key) {
         }
         final String[] keys = key.trim().split("\\|");
         final int keyLen = keys.length;
+        if (keyLen == 0) {
+            return null;
+        }
         final String subKey = keys[0].trim();
         final String subValue = keyLen < 2 ? key : keys[1].trim();
         try {
diff --git a/src/test/java/org/apache/commons/text/lookup/DnsStringLookupTest.java b/src/test/java/org/apache/commons/text/lookup/DnsStringLookupTest.java
index b62dee6aa5..a5c5f25248 100644
--- a/src/test/java/org/apache/commons/text/lookup/DnsStringLookupTest.java
+++ b/src/test/java/org/apache/commons/text/lookup/DnsStringLookupTest.java
@@ -73,6 +73,14 @@ void testName() throws UnknownHostException {
         assertTrue(matched);
     }
 
+    @Test
+    void testDelimiterOnlyKey() {
+        // A key that is only delimiter/whitespace splits to an empty array; must not throw.
+        assertNull(DnsStringLookup.INSTANCE.apply("|"));
+        assertNull(DnsStringLookup.INSTANCE.apply("||"));
+        assertNull(DnsStringLookup.INSTANCE.apply("  |  "));
+    }
+
     @Test
     void testNull() {
         assertNull(DnsStringLookup.INSTANCE.apply(null));