diff --git a/tools/check/predef.jam b/tools/check/predef.jam
index 6355ea7..c037fb4 100644
--- a/tools/check/predef.jam
+++ b/tools/check/predef.jam
@@ -43,7 +43,7 @@ rule check ( expressions + : language ? : true-properties * : false-properties *
         else
         {
             # Create the check run if we don't have one yet.
-            local key = [ MD5 $(language)::$(expression) ] ;
+            local key = [ MD5 "$(language)::$(expression)" ] ;
             if ! ( $(key) in $(_checks_) )
             {
                 _checks_ += $(key) ;