diff --git a/fco2/GenerateC.hs b/fco2/GenerateC.hs
index d6871ef..178cb14 100644
--- a/fco2/GenerateC.hs
+++ b/fco2/GenerateC.hs
@@ -498,7 +498,7 @@ genInputItem c (A.InCounted m cv av)
           tell [", "]
           fst $ abbrevVariable A.Abbrev t av
           tell [", "]
-          subT <- subscriptType (A.Subscript m $ makeConstant m 0) t
+          subT <- trivialSubscriptType t
           genVariable cv
           tell [" * "]
           genBytesInType subT
@@ -533,7 +533,7 @@ genOutputItem c (A.OutCounted m ce ae)
                  tell [", "]
                  fst $ abbrevVariable A.Abbrev t v
                  tell [", "]
-                 subT <- subscriptType (A.Subscript m $ makeConstant m 0) t
+                 subT <- trivialSubscriptType t
                  genExpression ce
                  tell [" * "]
                  genBytesInType subT
diff --git a/fco2/Parse.hs b/fco2/Parse.hs
index cda8d29..448a995 100644
--- a/fco2/Parse.hs
+++ b/fco2/Parse.hs
@@ -472,7 +472,7 @@ pushSubscriptTypeContext
     =  do ps <- get
           case psTypeContext ps of
             (Just t):_ ->
-              do subT <- subscriptType (A.Subscript emptyMeta $ makeConstant emptyMeta 0) t
+              do subT <- trivialSubscriptType t
                  pushTypeContext $ Just subT
             _ -> pushTypeContext Nothing
 --}}}
@@ -1200,7 +1200,7 @@ chanArrayAbbrev
                                  n <- newChannelName
                                  sIS
                                  sLeft
-                                 ct <- subscriptType (A.Subscript m $ makeConstant m 0) s
+                                 ct <- trivialSubscriptType s
                                  case ct of
                                    A.Chan _ -> return (ct, s, n)
                                    _ -> pzero)
diff --git a/fco2/Types.hs b/fco2/Types.hs
index d4ee818..6c58ccc 100644
--- a/fco2/Types.hs
+++ b/fco2/Types.hs
@@ -101,6 +101,15 @@ subscriptType (A.SubscriptField m tag) t = typeOfRecordField m t tag
 subscriptType (A.Subscript m sub) t = plainSubscriptType m sub t
 subscriptType _ _ = die "unsubscriptable type"
 
+-- | Just remove the first dimension from an array type -- like doing
+-- subscriptType with constant 0 as a subscript, but without the checking.
+-- This is used for the couple of cases where we know it's safe and don't want
+-- the usage check.
+trivialSubscriptType :: (Die m) => A.Type -> m A.Type
+trivialSubscriptType (A.Array [d] t) = return t
+trivialSubscriptType (A.Array (d:ds) t) = return $ A.Array ds t
+trivialSubscriptType _ = die "not plain array type"
+
 typeOfVariable :: (PSM m, Die m) => A.Variable -> m A.Type
 typeOfVariable (A.Variable m n) = typeOfName n
 typeOfVariable (A.SubscriptedVariable m s v)