diff --git a/db/queryutil.cpp b/db/queryutil.cpp
index f81beade4ef..4d45a3b6326 100644
--- a/db/queryutil.cpp
+++ b/db/queryutil.cpp
@@ -91,7 +91,7 @@ namespace mongo {
         }
     }
     
-    FieldBound &FieldBound::operator&=( const FieldBound &other ) {
+    const FieldBound &FieldBound::operator&=( const FieldBound &other ) {
         if ( other.upper_.woCompare( upper_, false ) < 0 )
             upper_ = other.upper_;
         if ( other.lower_.woCompare( lower_, false ) > 0 )
diff --git a/db/queryutil.h b/db/queryutil.h
index 5d98aacb5c5..a1533f6d004 100644
--- a/db/queryutil.h
+++ b/db/queryutil.h
@@ -31,7 +31,7 @@ namespace mongo {
     class FieldBound {
     public:
         FieldBound( const BSONElement &e = emptyObj.firstElement() );
-        FieldBound &operator&=( const FieldBound &other );
+        const FieldBound &operator&=( const FieldBound &other );
         BSONElement lower() const { return lower_; }
         BSONElement upper() const { return upper_; }
         bool equality() const { return lower_.woCompare( upper_, false ) == 0; }