diff --git a/src/main/wacc/parser.scala b/src/main/wacc/parser.scala
index 5751732..247ae31 100644
--- a/src/main/wacc/parser.scala
+++ b/src/main/wacc/parser.scala
@@ -110,8 +110,9 @@ object parser {
   private lazy val `<pair-elem-type>` =
     (`<base-type>` <**> (`<array-type>` </> identity)) |
       ((UntypedPairType from `<pair-type>`) <**>
-        ((`<pair-elems-type>` <**> `<array-type>`)
-          .map(arr => (_: UntypedPairType) => arr) </> identity))
+        ((`<pair-elems-type>` <**> `<array-type>`.explain(
+          "non-erased pair types cannot be nested"
+        )).map(arr => (_: UntypedPairType) => arr) </> identity))
 
   // Statements
   private lazy val `<program>` = Program(