1 files changed, 4 insertions, 1 deletions
diff --git a/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java b/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java
index 2a8e6bdf..fe0c6271 100644
--- a/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java
+++ b/src/main/java/org/shedlang/compiler/sources/FileFragmentSource.java
@@ -32,11 +32,14 @@ public record FileFragmentSource(
 
     @Override
     public Source at(int characterIndexStart, int characterIndexEnd) {
+        if (characterIndexStart > characterIndexEnd) {
+            throw new IllegalArgumentException("characterIndexStart > characterIndexEnd");
+        }
+
         return new FileFragmentSource(
             filename,
             contents,
             this.characterIndexStart + characterIndexStart,
-            // TODO: check this is less than characterIndexEnd
             this.characterIndexStart + characterIndexEnd
         );
     }