diff --git a/core/src/main/java/org/elasticsearch/search/suggest/completion/context/ContextMappings.java b/core/src/main/java/org/elasticsearch/search/suggest/completion/context/ContextMappings.java
index 0cac17dbb81..bbefbf41b89 100644
--- a/core/src/main/java/org/elasticsearch/search/suggest/completion/context/ContextMappings.java
+++ b/core/src/main/java/org/elasticsearch/search/suggest/completion/context/ContextMappings.java
@@ -81,7 +81,9 @@ public class ContextMappings implements ToXContent {
     public ContextMapping get(String name) {
         ContextMapping contextMapping = contextNameMap.get(name);
         if (contextMapping == null) {
-            throw new IllegalArgumentException("Unknown context name[" + name + "], must be one of " + contextNameMap.size());
+            List<String> keys = new ArrayList<>(contextNameMap.keySet());
+            Collections.sort(keys);
+            throw new IllegalArgumentException("Unknown context name [" + name + "], must be one of " + keys.toString());
         }
         return contextMapping;
     }
diff --git a/core/src/test/java/org/elasticsearch/search/suggest/completion/CategoryContextMappingTests.java b/core/src/test/java/org/elasticsearch/search/suggest/completion/CategoryContextMappingTests.java
index a9639c7b465..b9b15341da0 100644
--- a/core/src/test/java/org/elasticsearch/search/suggest/completion/CategoryContextMappingTests.java
+++ b/core/src/test/java/org/elasticsearch/search/suggest/completion/CategoryContextMappingTests.java
@@ -29,6 +29,7 @@ import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.common.xcontent.json.JsonXContent;
+import org.elasticsearch.index.mapper.CompletionFieldMapper.CompletionFieldType;
 import org.elasticsearch.index.mapper.DocumentMapper;
 import org.elasticsearch.index.mapper.FieldMapper;
 import org.elasticsearch.index.mapper.MappedFieldType;
@@ -673,6 +674,31 @@ public class CategoryContextMappingTests extends ESSingleNodeTestCase {
         Exception e = expectThrows(ElasticsearchParseException.class, () -> mapping.parseQueryContext(createParseContext(parser)));
         assertEquals("category context must be an object, string, number or boolean", e.getMessage());
     }
+    
+    public void testUnknownQueryContextParsing() throws Exception {
+        String mapping = jsonBuilder().startObject().startObject("type1")
+                .startObject("properties").startObject("completion")
+                .field("type", "completion")
+                .startArray("contexts")
+                .startObject()
+                .field("name", "ctx")
+                .field("type", "category")
+                .endObject()
+                .startObject()
+                .field("name", "type")
+                .field("type", "category")
+                .endObject()
+                .endArray()
+                .endObject().endObject()
+                .endObject().endObject().string();
+
+        DocumentMapper defaultMapper = createIndex("test").mapperService().documentMapperParser().parse("type1", new CompressedXContent(mapping));
+        FieldMapper fieldMapper = defaultMapper.mappers().getMapper("completion");
+        CompletionFieldType completionFieldType = (CompletionFieldType) fieldMapper.fieldType();
+        
+        Exception e = expectThrows(IllegalArgumentException.class, () -> completionFieldType.getContextMappings().get("brand"));
+        assertEquals("Unknown context name [brand], must be one of [ctx, type]", e.getMessage());
+    }
 
     public void testParsingContextFromDocument() throws Exception {
         CategoryContextMapping mapping = ContextBuilder.category("cat").field("category").build();