diff --git a/CHANGELOG.md b/CHANGELOG.md
index bc6340e6..6076b631 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased]
 
+### Fixed
+
+- Don't try to assign non-existing output writer during eval mode.
+
 ## [3.4.0] - 2023-06-19
 
 ### Added
diff --git a/casanovo/denovo/model_runner.py b/casanovo/denovo/model_runner.py
index fb5deeba..223c14c7 100644
--- a/casanovo/denovo/model_runner.py
+++ b/casanovo/denovo/model_runner.py
@@ -124,7 +124,7 @@ def _execute_existing(
     if len(peak_filenames := _get_peak_filenames(peak_path, peak_ext)) == 0:
         logger.error("Could not find peak files from %s", peak_path)
         raise FileNotFoundError("Could not find peak files")
-    else:
+    elif out_writer is not None:
         out_writer.set_ms_run(peak_filenames)
     peak_is_index = any(
         [os.path.splitext(fn)[1] in (".h5", ".hdf5") for fn in peak_filenames]