diff --git a/q2_types/feature_data/_format.py b/q2_types/feature_data/_format.py
index 408b1c57..bc8a0170 100644
--- a/q2_types/feature_data/_format.py
+++ b/q2_types/feature_data/_format.py
@@ -296,7 +296,7 @@ def _validate_lines(self, max_lines):
 
                     elif re.fullmatch(FASTADNAValidator, line):
                         if prev_seq_start_line == 0:
-                            prev_seq_start_line = line
+                            prev_seq_start_line = line_number
 
                         prev_seq_len += len(line)
                         last_line_was_ID = False