diff --git a/core/src/main/java/org/elasticsearch/index/mapper/core/StringFieldMapper.java b/core/src/main/java/org/elasticsearch/index/mapper/core/StringFieldMapper.java
index 65d11b45475..0c98dd10c3e 100644
--- a/core/src/main/java/org/elasticsearch/index/mapper/core/StringFieldMapper.java
+++ b/core/src/main/java/org/elasticsearch/index/mapper/core/StringFieldMapper.java
@@ -29,7 +29,6 @@ import org.apache.lucene.search.RegexpQuery;
 import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.Nullable;
-import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.logging.ESLogger;
 import org.elasticsearch.common.logging.Loggers;
@@ -204,6 +203,9 @@ public class StringFieldMapper extends FieldMapper implements AllFieldMapper.Inc
         public Mapper.Builder parse(String fieldName, Map<String, Object> node, ParserContext parserContext) throws MapperParsingException {
             if (parserContext.indexVersionCreated().onOrAfter(Version.V_5_0_0_alpha1)) {
                 final Object index = node.get("index");
+                if (Arrays.asList(null, "no", "not_analyzed", "analyzed").contains(index) == false) {
+                    throw new IllegalArgumentException("Can't parse [index] value [" + index + "] for field [" + fieldName + "], expected [no], [not_analyzed] or [analyzed]");
+                }
                 final boolean keyword = index != null && "analyzed".equals(index) == false;
 
                 // Automatically upgrade simple mappings for ease of upgrade, otherwise fail
@@ -283,7 +285,7 @@ public class StringFieldMapper extends FieldMapper implements AllFieldMapper.Inc
                     node.put("index", false);
                     break;
                 default:
-                    throw new IllegalArgumentException("Can't parse [index] value [" + index + "] for field [" + fieldName + "], expected [true], [false], [no], [not_analyzed] or [analyzed]");
+                    throw new IllegalArgumentException("Can't parse [index] value [" + index + "] for field [" + fieldName + "], expected [no], [not_analyzed] or [analyzed]");
                 }
             }
             final Object fielddataObject = node.get("fielddata");
diff --git a/core/src/test/java/org/elasticsearch/index/mapper/core/StringMappingUpgradeTests.java b/core/src/test/java/org/elasticsearch/index/mapper/core/StringMappingUpgradeTests.java
index a8dd1b65fcf..41094fec58a 100644
--- a/core/src/test/java/org/elasticsearch/index/mapper/core/StringMappingUpgradeTests.java
+++ b/core/src/test/java/org/elasticsearch/index/mapper/core/StringMappingUpgradeTests.java
@@ -103,6 +103,22 @@ public class StringMappingUpgradeTests extends ESSingleNodeTestCase {
         assertEquals(IndexOptions.NONE, field.fieldType().indexOptions());
     }
 
+    public void testIllegalIndexValue() throws IOException {
+        IndexService indexService = createIndex("test");
+        DocumentMapperParser parser = indexService.mapperService().documentMapperParser();
+        String mapping = XContentFactory.jsonBuilder().startObject().startObject("type")
+                .startObject("properties")
+                    .startObject("field")
+                        .field("type", "string")
+                        .field("index", false)
+                    .endObject()
+                .endObject() .endObject().endObject().string();
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class,
+                () -> parser.parse("type", new CompressedXContent(mapping)));
+        assertThat(e.getMessage(),
+                containsString("Can't parse [index] value [false] for field [field], expected [no], [not_analyzed] or [analyzed]"));
+    }
+
     public void testNotSupportedUpgrade() throws IOException {
         IndexService indexService = createIndex("test");
         DocumentMapperParser parser = indexService.mapperService().documentMapperParser();