diff --git a/SourceGen/LabelFileGenerator.cs b/SourceGen/LabelFileGenerator.cs
index b70ad1d..e8a8279 100644
--- a/SourceGen/LabelFileGenerator.cs
+++ b/SourceGen/LabelFileGenerator.cs
@@ -67,16 +67,21 @@ namespace SourceGen {
// Sort alphabetically. Not necessary, but it could make a "diff" easier to read.
symList.Sort((a, b) => Symbol.Compare(Symbol.SymbolSortField.Name, true, a, b));
+ Debug.Assert(mFormat == LabelFmt.VICE);
+
// VICE format is "add_label