diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,8 @@
+2022-01-07
+        * Version bump (3.7). (#287)
+        * Guard against empty specs. (#274)
+        * Make typetypes respect dependency order. (#275)
+
 2021-11-07
         * Version bump (3.6). (#264)
         * Introduce new ops atan2, ceiling, floor. (#246)
diff --git a/copilot-c99.cabal b/copilot-c99.cabal
--- a/copilot-c99.cabal
+++ b/copilot-c99.cabal
@@ -1,6 +1,6 @@
 cabal-version             : >= 1.10
 name                      : copilot-c99
-version                   : 3.6
+version                   : 3.7
 synopsis                  : A compiler for Copilot targeting C99.
 description               :
   This package is a back-end from Copilot to C.
@@ -48,7 +48,7 @@
                           , mtl                 >= 2.2 && < 2.3
                           , pretty              >= 1.1 && < 1.2
 
-                          , copilot-core        >= 3.6   && < 3.7
+                          , copilot-core        >= 3.7   && < 3.8
                           , language-c99        >= 0.1.1 && < 0.2
                           , language-c99-util   >= 0.1.1 && < 0.2
                           , language-c99-simple >= 0.1.1 && < 0.2
diff --git a/src/Copilot/Compile/C99/CodeGen.hs b/src/Copilot/Compile/C99/CodeGen.hs
--- a/src/Copilot/Compile/C99/CodeGen.hs
+++ b/src/Copilot/Compile/C99/CodeGen.hs
@@ -165,8 +165,8 @@
 -- | List all types of a type, returns items uniquely.
 typetypes :: Typeable a => Type a -> [UType]
 typetypes ty = case ty of
-  Array ty' -> [UType ty] `union` typetypes ty'
-  Struct x  -> [UType ty] `union` map (\(Value ty' _) -> UType ty') (toValues x)
+  Array ty' -> typetypes ty' `union` [UType ty]
+  Struct x  -> concatMap (\(Value ty' _) -> typetypes ty') (toValues x) `union` [UType ty]
   _         -> [UType ty]
 
 -- | Collect all expression of a list of streams and triggers and wrap them
diff --git a/src/Copilot/Compile/C99/Compile/Internal.hs b/src/Copilot/Compile/C99/Compile/Internal.hs
--- a/src/Copilot/Compile/C99/Compile/Internal.hs
+++ b/src/Copilot/Compile/C99/Compile/Internal.hs
@@ -8,7 +8,9 @@
 import Data.List            (nub)
 import Data.Maybe           (catMaybes)
 import System.Directory     (createDirectoryIfMissing)
+import System.Exit          (exitFailure)
 import System.FilePath      ((</>))
+import System.IO            (hPutStrLn, stderr)
 
 import Language.C99.Pretty  (pretty)
 import qualified Language.C99.Simple as C
@@ -26,27 +28,34 @@
 --
 -- The second argument is used as prefix for the .h and .c files generated.
 compileWith :: CSettings -> String -> Spec -> IO ()
-compileWith cSettings prefix spec = do
-  let cfile = render $ pretty $ C.translate $ compilec cSettings spec
-      hfile = render $ pretty $ C.translate $ compileh cSettings spec
+compileWith cSettings prefix spec
+  | null (specTriggers spec)
+  = do hPutStrLn stderr $
+         "Copilot error: attempt at compiling empty specification.\n"
+         ++ "You must define at least one trigger to generate C monitors."
+       exitFailure
 
-      -- TODO: find a nicer solution using annotated AST's
-      -- Should figure out exactly which headers are needed, based on what
-      -- is used.
-      cmacros = unlines [ "#include <stdint.h>"
-                        , "#include <stdbool.h>"
-                        , "#include <string.h>"
-                        , "#include <stdlib.h>"
-                        , "#include <math.h>"
-                        , ""
-                        , "#include \"" ++ prefix ++ ".h\""
-                        , ""
-                        ]
+  | otherwise
+  = do let cfile = render $ pretty $ C.translate $ compilec cSettings spec
+           hfile = render $ pretty $ C.translate $ compileh cSettings spec
 
-  let dir = cSettingsOutputDirectory cSettings
-  createDirectoryIfMissing True dir
-  writeFile (dir </> prefix ++ ".c") $ cmacros ++ cfile
-  writeFile (dir </> prefix ++ ".h") hfile
+           -- TODO: find a nicer solution using annotated AST's
+           -- Should figure out exactly which headers are needed, based on what
+           -- is used.
+           cmacros = unlines [ "#include <stdint.h>"
+                             , "#include <stdbool.h>"
+                             , "#include <string.h>"
+                             , "#include <stdlib.h>"
+                             , "#include <math.h>"
+                             , ""
+                             , "#include \"" ++ prefix ++ ".h\""
+                             , ""
+                             ]
+
+       let dir = cSettingsOutputDirectory cSettings
+       createDirectoryIfMissing True dir
+       writeFile (dir </> prefix ++ ".c") $ cmacros ++ cfile
+       writeFile (dir </> prefix ++ ".h") hfile
 
 -- | Compile a specification to a .h and a .c file.
 --