crash in case of invalid api usage

api.lua

@@ -5,8 +5,16 @@ areas.registered_on_adds = {}
 areas.registered_on_removes = {}
 areas.registered_on_moves = {}
 
+areas.callback_origins = {}
+
 function areas:registerProtectionCondition(func)
 	table.insert(areas.registered_protection_conditions, func)
+	local debug_info = debug.getinfo(func, "S")
+	areas.callback_origins[func] = {
+		mod = core.get_current_modname() or "??",
+		source = debug_info.short_src or "??",
+		line = debug_info.linedefined or "??"
+	}
 end
 
 function areas:registerOnAdd(func)

internal.lua

@@ -222,6 +222,11 @@ function areas:canPlayerAddArea(pos1, pos2, name)
 			allowed = false
 			-- save the first error that occurred
 			errMsg = errMsg or msg
+		elseif res ~= nil then
+			local origin = areas.callback_origins[areas.registered_protection_conditions[i]]
+			error("\n[Mod] areas: Invalid api usage from mod '" ..
+					origin.mod .. "' in callback registerProtectionCondition() at " ..
+					origin.source .. ":" .. origin.line)
 		end
 	end