diff --git a/docs/LanguageGuide.md b/docs/LanguageGuide.md
index 329e26f6..590d8246 100644
--- a/docs/LanguageGuide.md
+++ b/docs/LanguageGuide.md
@@ -237,6 +237,8 @@ Note that match works with *values* (not references) takes ownership over the va
 
 Note that this code would not take ownership over `might-be-a-string`. Also, the `s` in the first case is a reference, since it wouldn't be safe to destructure the `Maybe` into values in this situation.
 
+**Note:** A sumtype cannot have more than 128 inhabitants, also known as constructors. If that reads to you like a byte limitation, you’re on the right track. While this is a limitation, it has not proved to be a problem as of yet.
+
 ### Modules and Name Lookup
 Functions and variables can be stored in modules which are named and can be nested. To use a symbol inside a module
 you need to qualify it with the module name, like this: `Float.cos`.
diff --git a/src/PrimitiveError.hs b/src/PrimitiveError.hs
index 4c992d78..96205044 100644
--- a/src/PrimitiveError.hs
+++ b/src/PrimitiveError.hs
@@ -19,6 +19,7 @@ data PrimitiveError
   | StructNotFound XObj
   | NonTypeInTypeEnv SymPath XObj
   | InvalidSumtypeCase XObj
+  | TooManySumtypeCases
 
 data PrimitiveWarning
   = NonExistentInterfaceWarning XObj
@@ -74,6 +75,8 @@ instance Show PrimitiveError where
   show (PrimitiveError.InvalidSumtypeCase xobj) =
     "Can't get members for an invalid sumtype case: "
       ++ pretty xobj
+  show TooManySumtypeCases =
+    "Got too many sumtype cases (>128) for type"
 
 instance Show PrimitiveWarning where
   show (NonExistentInterfaceWarning x) =
diff --git a/src/Primitives.hs b/src/Primitives.hs
index 675ed3af..4e914984 100644
--- a/src/Primitives.hs
+++ b/src/Primitives.hs
@@ -413,7 +413,7 @@ primitiveMembers _ ctx xobj@(XObj (Sym path _) _ _) =
     go (XObj (Lst [(XObj (Deftype _) _ _), _, (XObj (Arr members) _ _)]) _ _) =
       pure (ctx, Right (XObj (Arr (map (\(a, b) -> XObj (Lst [a, b]) Nothing Nothing) (pairwise members))) Nothing Nothing))
     go (XObj (Lst ((XObj (DefSumtype _) _ _) : _ : cases)) _ _) =
-      pure $ (ctx, (either Left (\a -> Right (XObj (Arr (concat a)) Nothing Nothing)) (mapM getMembersFromCase cases)))
+      pure (ctx, (either Left (\a -> Right (XObj (Arr (concat a)) Nothing Nothing)) (mapM getMembersFromCase cases)))
     go x = pure (toEvalError ctx x (NonTypeInTypeEnv path x))
     getMembersFromCase :: XObj -> Either EvalError [XObj]
     getMembersFromCase (XObj (Lst members) _ _) =
@@ -576,8 +576,10 @@ primitiveDeftype xobj ctx (name : rest@(XObj (Arr a) _ _ : _)) =
       if all isUnqualifiedSym objs
         then deftype ctx name (selectConstructor rest)
         else pure (toEvalError ctx xobj (QualifiedTypeMember rest))
-primitiveDeftype _ ctx (name : rest) =
-  deftype ctx name (selectConstructor rest)
+primitiveDeftype x ctx (name : rest) =
+  if length rest > 128
+    then pure (toEvalError ctx x TooManySumtypeCases)
+    else deftype ctx name (selectConstructor rest)
 primitiveDeftype _ _ _ = error "primitivedeftype"
 
 type ModuleCreator = Context -> String -> [Ty] -> [XObj] -> Maybe Info -> Either TypeError (String, XObj, [XObj])