diff --git a/scripting/engine_spidermonkey.cpp b/scripting/engine_spidermonkey.cpp
index 6267a3fa545..5d66f15555d 100644
--- a/scripting/engine_spidermonkey.cpp
+++ b/scripting/engine_spidermonkey.cpp
@@ -219,7 +219,7 @@ namespace mongo {
                 ( (boost::uint64_t)(boost::uint32_t)getNumber( o , "top" ) << 32 ) +
                 ( boost::uint32_t)( getNumber( o , "bottom" ) );
             } else {
-                val = (boost::uint64_t) getNumber( o, "floatApprox" );
+                val = (boost::uint64_t)(boost::int64_t) getNumber( o, "floatApprox" );
             }
             return val;
         }