diff --git a/src/protocol/ast.rs b/src/protocol/ast.rs
index 468330bceee9955aaa86445979d345f962df61e9..63762b6dadd95850af6bb4b2325310725ccb67ea 100644
--- a/src/protocol/ast.rs
+++ b/src/protocol/ast.rs
@@ -372,6 +372,10 @@ pub enum ParserTypeVariant {
     Array,
     Input,
     Output,
+    // Tuple: expecting any number of elements. Note that the parser type can
+    // have one-valued tuples, these will be filtered out later during type
+    // checking.
+    Tuple(u32), // u32 = number of subsequent types
     // User-defined types
     PolymorphicArgument(DefinitionId, u32), // u32 = index into polymorphic variables
     Definition(DefinitionId, u32), // u32 = number of subsequent types in the type tree.
@@ -391,7 +395,7 @@ impl ParserTypeVariant {
                 0,
             ArrayLike | InputOrOutput | Array | Input | Output =>
                 1,
-            Definition(_, num) => *num as usize,
+            Definition(_, num) | Tuple(num) => *num as usize,
         }
     }
 }