diff --git a/core/src/main/java/org/elasticsearch/action/support/AutoCreateIndex.java b/core/src/main/java/org/elasticsearch/action/support/AutoCreateIndex.java
index 5c9152b4751..a9a5afed9f3 100644
--- a/core/src/main/java/org/elasticsearch/action/support/AutoCreateIndex.java
+++ b/core/src/main/java/org/elasticsearch/action/support/AutoCreateIndex.java
@@ -111,9 +111,10 @@ public final class AutoCreateIndex {
                 try {
                     String[] patterns = Strings.commaDelimitedListToStringArray(value);
                     for (String pattern : patterns) {
-                        if (pattern == null || pattern.length() == 0) {
+                        if (pattern == null || pattern.trim().length() == 0) {
                             throw new IllegalArgumentException("Can't parse [" + value + "] for setting [action.auto_create_index] must be either [true, false, or a comma separated list of index patterns]");
                         }
+                        pattern = pattern.trim();
                         Tuple<String, Boolean> expression;
                         if (pattern.startsWith("-")) {
                             if (pattern.length() == 1) {
diff --git a/core/src/test/java/org/elasticsearch/action/support/AutoCreateIndexTests.java b/core/src/test/java/org/elasticsearch/action/support/AutoCreateIndexTests.java
index 8a45ca47535..19342e5e604 100644
--- a/core/src/test/java/org/elasticsearch/action/support/AutoCreateIndexTests.java
+++ b/core/src/test/java/org/elasticsearch/action/support/AutoCreateIndexTests.java
@@ -25,11 +25,16 @@ import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
 import org.elasticsearch.cluster.metadata.MetaData;
+import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.test.ESTestCase;
 
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
 import static org.hamcrest.CoreMatchers.equalTo;
 
 public class AutoCreateIndexTests extends ESTestCase {
@@ -57,6 +62,24 @@ public class AutoCreateIndexTests extends ESTestCase {
         }
     }
 
+    public void testHandleSpaces() { // see #21449
+        Settings settings = Settings.builder().put(AutoCreateIndex.AUTO_CREATE_INDEX_SETTING.getKey(),
+            randomFrom(".marvel-, .security, .watches, .triggered_watches, .watcher-history-",
+                ".marvel-,.security,.watches,.triggered_watches,.watcher-history-")).build();
+        AutoCreateIndex autoCreateIndex = newAutoCreateIndex(settings);
+        List<Tuple<String, Boolean>> expressions = autoCreateIndex.getAutoCreate().getExpressions();
+        Map<String, Boolean> map = new HashMap<>();
+        for (Tuple<String, Boolean> t : expressions) {
+            map.put(t.v1(), t.v2());
+        }
+        assertTrue(map.get(".marvel-"));
+        assertTrue(map.get(".security"));
+        assertTrue(map.get(".watches"));
+        assertTrue(map.get(".triggered_watches"));
+        assertTrue(map.get(".watcher-history-"));
+        assertEquals(5, map.size());
+    }
+
     public void testAutoCreationDisabled() {
         Settings settings = Settings.builder().put(AutoCreateIndex.AUTO_CREATE_INDEX_SETTING.getKey(), false).build();
         AutoCreateIndex autoCreateIndex = newAutoCreateIndex(settings);