diff --git a/mef/schema/expressions.rnc b/mef/schema/expressions.rnc
index d0a65ce..7c48886 100644
--- a/mef/schema/expressions.rnc
+++ b/mef/schema/expressions.rnc
@@ -32,7 +32,7 @@ constant = bool | int | float
 
 bool =
   element bool {
-    attribute value { Boolean-value }
+    attribute value { xsd:boolean }
   }
 
 int =
diff --git a/mef/schema/formula.rnc b/mef/schema/formula.rnc
index 9d522ea..faba1d8 100644
--- a/mef/schema/formula.rnc
+++ b/mef/schema/formula.rnc
@@ -38,7 +38,5 @@ basic-event = element basic-event { reference }
 
 Boolean-constant =
   element constant {
-    attribute value { Boolean-value }
+    attribute value { xsd:boolean }
   }
-
-Boolean-value = "true" | "false"