diff --git a/db/pdfile.cpp b/db/pdfile.cpp
index e7e10bbf1f1..eab698ecf06 100644
--- a/db/pdfile.cpp
+++ b/db/pdfile.cpp
@@ -1750,7 +1750,7 @@ namespace mongo {
             BSONObj io((const char *) obuf);
             BSONElement idField = io.getField( "_id" );
             uassert( 10099 ,  "_id cannot be an array", idField.type() != Array );
-            if( idField.eoo() && !wouldAddIndex && strstr(ns, ".local.") == 0 ) {
+            if( idField.eoo() && !wouldAddIndex && strstr(ns, ".local.") == 0 && !d->capped) {
                 if( addedID )
                     *addedID = true;
                 addID = len;
diff --git a/db/query.cpp b/db/query.cpp
index 0d29ddeb10d..7f1b67887d5 100644
--- a/db/query.cpp
+++ b/db/query.cpp
@@ -1083,6 +1083,10 @@ namespace mongo {
         }
 
         if ( ! (explain || pq.showDiskLoc()) && isSimpleIdQuery( query ) && !pq.hasOption( QueryOption_CursorTailable ) ) {
+
+			NamespaceDetails* d = nsdetails(ns);
+			uassert(14815, "capped collections have no _id index by default", !(d && d->capped));
+
             bool nsFound = false;
             bool indexFound = false;
 
diff --git a/dbtests/namespacetests.cpp b/dbtests/namespacetests.cpp
index 513d90a938f..392917dd6d3 100644
--- a/dbtests/namespacetests.cpp
+++ b/dbtests/namespacetests.cpp
@@ -625,9 +625,11 @@ namespace NamespaceTests {
             NamespaceDetails *nsd() const {
                 return nsdetails( ns() )->writingWithExtra();
             }
-            static BSONObj bigObj() {
-                string as( 187, 'a' );
+            static BSONObj bigObj(bool bGenID=false) {
                 BSONObjBuilder b;
+				if (bGenID)
+					b.appendOID("_id", 0, true);
+                string as( 187, 'a' );
                 b.append( "a", as );
                 return b.obj();
             }
@@ -660,12 +662,12 @@ namespace NamespaceTests {
         public:
             void run() {
                 create();
-                BSONObj b = bigObj();
 
                 const int N = 20;
                 const int Q = 16; // these constants depend on the size of the bson object, the extent size allocated by the system too
                 DiskLoc l[ N ];
                 for ( int i = 0; i < N; ++i ) {
+					BSONObj b = bigObj(true);
                     l[ i ] = theDataFileMgr.insert( ns(), b.objdata(), b.objsize() );
                     ASSERT( !l[ i ].isNull() );
                     ASSERT( nRecords() <= Q );
@@ -717,7 +719,7 @@ namespace NamespaceTests {
                 create();
                 ASSERT_EQUALS( 2, nExtents() );
 
-                BSONObj b = bigObj();
+                BSONObj b = bigObj(true);
 
                 int N = MinExtentSize / b.objsize() * nExtents() + 5;
                 int T = N - 4;
@@ -725,7 +727,8 @@ namespace NamespaceTests {
                 DiskLoc truncAt;
                 //DiskLoc l[ 8 ];
                 for ( int i = 0; i < N; ++i ) {
-                    DiskLoc a = theDataFileMgr.insert( ns(), b.objdata(), b.objsize() );
+					BSONObj bb = bigObj(true);
+                    DiskLoc a = theDataFileMgr.insert( ns(), bb.objdata(), bb.objsize() );
                     if( T == i )
                         truncAt = a;
                     ASSERT( !a.isNull() );
@@ -765,6 +768,7 @@ namespace NamespaceTests {
 
                 // Too big
                 BSONObjBuilder bob;
+				bob.appendOID("_id", 0, true);
                 bob.append( "a", string( MinExtentSize + 300, 'a' ) );
                 BSONObj bigger = bob.done();
                 ASSERT( theDataFileMgr.insert( ns(), bigger.objdata(), bigger.objsize() ).isNull() );
diff --git a/dbtests/querytests.cpp b/dbtests/querytests.cpp
index 2fcb2a1deef..e208c995173 100644
--- a/dbtests/querytests.cpp
+++ b/dbtests/querytests.cpp
@@ -355,12 +355,20 @@ namespace QueryTests {
         ~TailableQueryOnId() {
             client().dropCollection( "unittests.querytests.TailableQueryOnId" );
         }
+
+		void insertA(const char* ns, int a) {
+			BSONObjBuilder b;
+			b.appendOID("_id", 0, true);
+			b.append("a", a);
+			insert(ns, b.obj());
+		}
+
         void run() {
             const char *ns = "unittests.querytests.TailableQueryOnId";
             BSONObj info;
             client().runCommand( "unittests", BSON( "create" << "querytests.TailableQueryOnId" << "capped" << true << "autoIndexId" << true ), info );
-            insert( ns, BSON( "a" << 0 ) );
-            insert( ns, BSON( "a" << 1 ) );
+            insertA( ns, 0 );
+            insertA( ns, 1 );
             auto_ptr< DBClientCursor > c1 = client().query( ns, QUERY( "a" << GT << -1 ), 0, 0, 0, QueryOption_CursorTailable );
             OID id;
             id.init("000000000000000000000000");
@@ -371,7 +379,7 @@ namespace QueryTests {
             c2->next();
             c2->next();
             ASSERT( !c2->more() );
-            insert( ns, BSON( "a" << 2 ) );
+            insertA( ns, 2 );
             ASSERT( c1->more() );
             ASSERT_EQUALS( 2, c1->next().getIntField( "a" ) );
             ASSERT( !c1->more() );
@@ -880,7 +888,10 @@ namespace QueryTests {
         }
 
         void insertNext() {
-            insert( ns() , BSON( "i" << _n++ ) );
+			BSONObjBuilder b;
+			b.appendOID("_id", 0, true);
+			b.append("i", _n++);
+            insert( ns() , b.obj() );
         }
 
         int _n;