diff --git a/src/ConfigParser.cs b/src/ConfigParser.cs
index 2bd2c96..5fb1a1b 100644
--- a/src/ConfigParser.cs
+++ b/src/ConfigParser.cs
@@ -422,7 +422,8 @@ public bool SetValue(string sectionName, string keyName, string value)
 
             if (!sections.TryGetValue(sectionName, out var section))
             {
-                section = new ConfigSection(sectionName, Lines.Max(l => l.LineNumber));
+                var lineNumber = (null != Lines && Lines.Any()) ? Lines.Max(l => l.LineNumber) : -1;
+                section = new ConfigSection(sectionName, lineNumber);
                 sections.Add(sectionName, section);
             }