diff --git a/lucene/contrib/memory/src/java/org/apache/lucene/index/memory/MemoryIndex.java b/lucene/contrib/memory/src/java/org/apache/lucene/index/memory/MemoryIndex.java
index 861d07c..4801a39 100644
--- a/lucene/contrib/memory/src/java/org/apache/lucene/index/memory/MemoryIndex.java
+++ b/lucene/contrib/memory/src/java/org/apache/lucene/index/memory/MemoryIndex.java
@@ -798,6 +798,8 @@ public class MemoryIndex {
 
       @Override
       public Terms terms(final String field) {
+        if (field == null)
+          return null;
         int i = Arrays.binarySearch(sortedFields, field, termComparator);
         if (i < 0) {
           return null;