diff --git a/runtime-jvm/main/src/main/scala/BuiltinTypes.scala b/runtime-jvm/main/src/main/scala/BuiltinTypes.scala
index 1667133a1..01041ed95 100644
--- a/runtime-jvm/main/src/main/scala/BuiltinTypes.scala
+++ b/runtime-jvm/main/src/main/scala/BuiltinTypes.scala
@@ -95,7 +95,7 @@ object BuiltinTypes {
     }
   }
 
-  def dataConstructorish(req: (Array[Value]) => Value, decompile: Term,
+  def dataConstructorish(req: Array[Value] => Value, decompile: Term,
                          paramNames: Name*): Computation = {
     val arity = paramNames.length
     val body: Computation = arity match {
@@ -119,7 +119,7 @@ object BuiltinTypes {
         (r,rec,top,stackU,x1,x0,stackB,x1b,x0b) => {
           // arity = 3, argsStart = top.toInt,     argsStop = top.toInt + 1
           // arity = 4, argsStart = top.toInt - 1, argsStop = top.toInt + 1
-          val argsStart = top.toInt - ((arity - compilation.K) + 1)
+          val argsStart = top.toInt + 1 - (arity - compilation.K)
           val argsStop = top.toInt + 1
           val args = new Array[Value](arity)
           locally {
diff --git a/unison-src/errors/poor-error-message/overapplied-data-constructor-loc.u b/unison-src/errors/poor-error-message/overapplied-data-constructor-loc.u
new file mode 100644
index 000000000..b45cd5c36
--- /dev/null
+++ b/unison-src/errors/poor-error-message/overapplied-data-constructor-loc.u
@@ -0,0 +1,17 @@
+-- board piece
+type P = X | O | E
+
+type Board = Board P P
+
+use Board.Board
+use P O X E
+
+case Board X O X
+  of Board a b c -> a
+
+
+-- gives this error:
+  -- This looks like a function call, but with a Board where the function should be.  Are you missing an operator?
+  --                                             ^^^^^
+  --    13 | case Board X O X
+  --              ^^^^^
diff --git a/unison-src/tests/tictactoe.uu b/unison-src/tests/tictactoe.u
similarity index 100%
rename from unison-src/tests/tictactoe.uu
rename to unison-src/tests/tictactoe.u
diff --git a/unison-src/tests/tictactoe0-array-oob1.u b/unison-src/tests/tictactoe0-array-oob1.u
new file mode 100644
index 000000000..462544dc9
--- /dev/null
+++ b/unison-src/tests/tictactoe0-array-oob1.u
@@ -0,0 +1,12 @@
+-- board piece
+
+type Board = Board UInt64 UInt64 UInt64
+
+use Board.Board
+
+-- uncommenting these gives errors from NPE to array index out of bounds -1, -2
+-- x = 1
+-- y = 2
+
+case Board 77 88 99
+  of Board a b c -> c
diff --git a/unison-src/tests/tictactoe0-npe.u b/unison-src/tests/tictactoe0-npe.u
new file mode 100644
index 000000000..a39e1280c
--- /dev/null
+++ b/unison-src/tests/tictactoe0-npe.u
@@ -0,0 +1,17 @@
+-- board piece
+type P = X | O | E
+
+type Board = Board P P P P P P P P P
+
+use Board.Board
+use P O X E
+
+whatevs a b c = a
+
+b = Board X O X O X X O E X
+x = 1
+y = 2
+z = 3
+
+case b of
+  Board a b c d e f g h i -> a
diff --git a/unison-src/tests/tictactoe0.u b/unison-src/tests/tictactoe0.u
new file mode 100644
index 000000000..3af01038b
--- /dev/null
+++ b/unison-src/tests/tictactoe0.u
@@ -0,0 +1,43 @@
+-- board piece
+type P = X | O | E
+
+type Board = Board P P P P P P P P P
+
+use Board Board
+use P O X E
+use Optional Some None
+
+orElse a b =
+  case a of
+    None -> b
+    a -> a
+
+namespace P where
+  (/=) : P -> P -> Boolean
+  a /= b = not (a == b)
+  (==) : P -> P -> Boolean
+  a == b = case (a,b) of
+    (X,X) -> true
+    (O,O) -> true
+    _ -> false
+
+
+b = (Board X O X
+           O X X
+           O E X)
+
+isWin board =
+  same : P -> P -> P -> Optional P
+  same a b c = if and (and (a P.== b) (a P.== c)) (a P./= E)
+               then Some a
+               else None
+  case board of
+    -- vertical top/center/bottom
+    -- horizontal left/center/right
+    -- diagonal rising/falling
+    Board a b c
+          d e f
+          g h i -> (same a b c)
+
+isWin b
+-- Some 3
diff --git a/unison-src/tests/tictactoe2.uu b/unison-src/tests/tictactoe2.u
similarity index 99%
rename from unison-src/tests/tictactoe2.uu
rename to unison-src/tests/tictactoe2.u
index d29720d85..e6df64228 100644
--- a/unison-src/tests/tictactoe2.uu
+++ b/unison-src/tests/tictactoe2.u
@@ -61,4 +61,3 @@ isWin board =
 isWin (Board X O X
              O X X
              O E X)
-