diff --git a/src/rstd/collections/pair.rs b/src/rstd/collections/pair.rs
index 8a38fe5..19df825 100644
--- a/src/rstd/collections/pair.rs
+++ b/src/rstd/collections/pair.rs
@@ -43,10 +43,11 @@ impl<ErrorA: Error, ErrorB: Error> Display for PairParseError<ErrorA, ErrorB> {
 
 impl<ErrorA: Error, ErrorB: Error> Error for PairParseError<ErrorA, ErrorB> {}
 
-impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>, B: Mentionable<'a, Ctx>> StaticPair<'a, Ctx>
-    for (A, B)
+impl<'a, Ctx: Context<'a>, A: MentionableBase<'a, Ctx>, B: MentionableBase<'a, Ctx>>
+    StaticPair<'a, Ctx> for (A, B)
 where
     A::Fctr: InlineableFactory<'a, Ctx>,
+    B::Fctr: ParseMode,
 {
     type FactoryData = (Self::FA, Self::FB);
     type A = A;