diff --git a/db/matcher.cpp b/db/matcher.cpp
index 8bc76e5b404..55ddd302a7c 100644
--- a/db/matcher.cpp
+++ b/db/matcher.cpp
@@ -80,6 +80,7 @@ namespace mongo {
     public:
         Where() {
             jsScope = 0;
+            func = 0;
         }
         ~Where() {
 
@@ -553,6 +554,7 @@ namespace mongo {
             where->scope->setBoolean( "fullObject" , true ); // this is a hack b/c fullObject used to be relevant
             
             int err = where->scope->invoke( where->func , BSONObj() , 1000 * 60 );
+            where->scope->setThis( 0 );
             if ( err == -3 ) { // INVOKE_ERROR
                 stringstream ss;
                 ss << "error on invocation of $where function:\n" 
diff --git a/scripting/engine_spidermonkey.cpp b/scripting/engine_spidermonkey.cpp
index 666b690a570..d78a4c90aa3 100644
--- a/scripting/engine_spidermonkey.cpp
+++ b/scripting/engine_spidermonkey.cpp
@@ -772,6 +772,7 @@ namespace mongo {
         Convertor c( cx );
 
         BSONHolder * holder = GETHOLDER( cx , obj );
+        assert( holder );
         holder->check();
         
         string s = c.toString( id );
@@ -1090,11 +1091,15 @@ namespace mongo {
 
         void setThis( const BSONObj * obj ){
             smlock;
-            if ( _this )
+            if ( _this ){
                 JS_RemoveRoot( _context , &_this );
+                _this = 0;
+            }
             
-            _this = _convertor->toJSObject( obj );
-            JS_AddNamedRoot( _context , &_this , "scope this" );
+            if ( obj ){
+                _this = _convertor->toJSObject( obj );
+                JS_AddNamedRoot( _context , &_this , "scope this" );
+            }
         }
 
         // ---- functions -----