diff --git a/src/toon_format/decoder.py b/src/toon_format/decoder.py
index 90f0849..26f33e8 100644
--- a/src/toon_format/decoder.py
+++ b/src/toon_format/decoder.py
@@ -156,12 +156,15 @@ def parse_header(
         delimiter = COMMA
         length_str = bracket_content[:-1]
 
-    # Parse length
+    # Parse length - reject non-integer strings (negative values are invalid)
     try:
         length = int(length_str)
     except ValueError:
         return None
 
+    if length < 0:
+        return None
+
     # Check for fields segment
     fields = None
     after_bracket = line[bracket_end + 1 :].strip()