diff --git a/assembler-8608.lua b/assembler-8608.lua
index f05b306..7b308a6 100644
--- a/assembler-8608.lua
+++ b/assembler-8608.lua
@@ -159,7 +159,8 @@ local function assembleCode(code, instrs)
 end
 
 local function readFile(fn)
-	local fi = io.open(fn, "r") or error("could not open file "..fn)
+	local fi, err = io.open(fn, "r")
+	if not fi then error("could not open file "..fn..": "..err) end
 	local text = fi:read("*a")
 	fi:close()
 	return text