diff --git a/lips/Preproc.lua b/lips/Preproc.lua
index 78308af..469b4d5 100644
--- a/lips/Preproc.lua
+++ b/lips/Preproc.lua
@@ -1,4 +1,5 @@
 local abs = math.abs
+local format = string.format
 local insert = table.insert
 
 local path = string.gsub(..., "[^.]+$", "")
@@ -13,6 +14,13 @@ function Preproc:init(options)
     self.options = options or {}
 end
 
+function Preproc:error(msg, got)
+    if got ~= nil then
+        msg = msg..', got '..tostring(got)
+    end
+    error(format('%s:%d: Error: %s', self.fn, self.line, msg), 2)
+end
+
 function Preproc:iter(statements)
     assert(statements)
     local i = 0