diff --git a/copilot-core.cabal b/copilot-core.cabal
--- a/copilot-core.cabal
+++ b/copilot-core.cabal
@@ -1,6 +1,6 @@
 cabal-version:       >=1.10
 name:                copilot-core
-version:             0.2.2
+version:             0.2.3
 synopsis:            An intermediate representation for Copilot.
 description:         Intermediate representation for Copilot.
                      Strictly follows Haskell 2010 except for universal
diff --git a/src/Copilot/Core/Interpret/Eval.hs b/src/Copilot/Core/Interpret/Eval.hs
--- a/src/Copilot/Core/Interpret/Eval.hs
+++ b/src/Copilot/Core/Interpret/Eval.hs
@@ -339,18 +339,21 @@
   Trigger
     { triggerGuard = e
     , triggerArgs  = args
-    } = take k $ map tag (zip bs vs) ++ repeat Nothing -- there might be 0 args!
+    } = map tag (zip bs vs) 
 
   where
   tag :: (Bool, a) -> Maybe a
   tag (True,  x) = Just x
   tag (False, _) = Nothing
 
+  -- Is the guard true?
   bs :: [Bool]
   bs = evalExprs_ k e strms
 
+  -- The argument outputs.
   vs :: [[Output]]
-  vs = transpose $ map evalUExpr args 
+  vs = if null args then replicate k []  -- might be 0 args.
+         else transpose $ map evalUExpr args
 
   evalUExpr :: UExpr -> [Output]
   evalUExpr (UExpr t e1) =