diff --git a/Services/RuleService.cs b/Services/RuleService.cs
index ac665b76c80a04cd6bdb3da456f26e7dcf171be8..24fafe089ef51697cfd2540cd1bf4b395a0a0985 100644
--- a/Services/RuleService.cs
+++ b/Services/RuleService.cs
@@ -40,7 +40,8 @@ namespace anydata.Services
         {
             foreach (var code in codes)
             {
-                formula = Regex.Replace(formula, $@"\b{Regex.Escape(code)}\b", data[code]?.ToString() ?? string.Empty);
+                var value = string.IsNullOrEmpty(data[code]?.ToString()) ? "null" : data[code].ToString();
+                formula = Regex.Replace(formula, $@"\b{Regex.Escape(code)}\b", value);
             }
 
             return formula;