diff --git a/db/pdfile.cpp b/db/pdfile.cpp
index 9cacfdfed44..fffa0bf74ff 100644
--- a/db/pdfile.cpp
+++ b/db/pdfile.cpp
@@ -1144,7 +1144,9 @@ assert( !eloc.isNull() );
                Note that btree buckets which we insert aren't BSONObj's, but in that case god==true.
             */
             BSONObj io((const char *) obuf);
-            if( !io.hasField("_id") && !addIndex && strstr(ns, ".local.") == 0 ) {
+            BSONElement idField = io.getField( "_id" );
+            uassert( "_id cannot not be an array", idField.type() != Array );
+            if( idField.eoo() && !addIndex && strstr(ns, ".local.") == 0 ) {
                 addID = len;
                 if ( writeId.eoo() ) {
                     // Very likely we'll add this elt, so little harm in init'ing here.
diff --git a/dbtests/querytests.cpp b/dbtests/querytests.cpp
index 3602fd25697..bd95c45f286 100644
--- a/dbtests/querytests.cpp
+++ b/dbtests/querytests.cpp
@@ -322,6 +322,20 @@ namespace QueryTests {
         }
     };
     
+    class ArrayId : public ClientBase {
+    public:
+        ~ArrayId() {
+            client().dropCollection( "querytests.ArrayId" );
+        }
+        void run() {
+            const char *ns = "querytests.ArrayId";
+            client().ensureIndex( ns, BSON( "_id" << 1 ) );
+            ASSERT( !error() );
+            client().insert( ns, fromjson( "{'_id':[1,2]}" ) );
+            ASSERT( error() );
+        }
+    };
+    
     class All : public UnitTest::Suite {
     public:
         All() {
@@ -338,6 +352,7 @@ namespace QueryTests {
             add< TailableDelete >();
             add< TailableInsertDelete >();
             add< OplogReplayMode >();
+            add< ArrayId >();
         }
     };