Error out if a template expansion returns nil (as it's normally an error).

diff --git a/first/ackbuilder.lua b/first/ackbuilder.lua
index a799c8a..539b348 100644 (file)
--- a/first/ackbuilder.lua
+++ b/first/ackbuilder.lua
@@ -256,7 +256,11 @@ local function templateexpand(list, vars)
                                if e then
                                        error(string.format("error evaluating expression: %s", e))
                                end
-                               return asstring(chunk())
+                               local value = chunk()
+                               if (value == nil) then
+                                       error(string.format("template expression expands to nil (probably an undefined variable)"))
+                               end
+                               return asstring(value)
                        end
                )
        end