2 files changed, 4 insertions, 1 deletions

diff --git a/src/headers.lua b/src/headers.lua
index f8b8ef8..004ef37 100644
--- a/src/headers.lua
+++ b/src/headers.lua
@@ -36,6 +36,9 @@ setmetatable(_M.canonic, {
 -- adds a header with a given canonical capitalization, e.g. for headers
 -- whose capitalization titlecase(header) would not reproduce correctly
 function _M.setcanonic(header)
+    if type(header) ~= "string" then
+        error("header must be a string", 2)
+    end
     _M.canonic[header:lower()] = header
 end
 
diff --git a/test/headerstest.lua b/test/headerstest.lua
index 1387fc7..cbf638f 100644
--- a/test/headerstest.lua
+++ b/test/headerstest.lua
@@ -103,7 +103,7 @@ local check_canonic = function(lower, expected)
     if got ~= expected then
         print("canonic[" .. lower .. "] = " .. tostring(got) ..
             ", expected " .. expected)
-        os.exit()
+        os.exit(1)
     end
 end