diff --git a/aider/coders/base_coder.py b/aider/coders/base_coder.py
index ca3a2fdef..387f1b33a 100755
--- a/aider/coders/base_coder.py
+++ b/aider/coders/base_coder.py
@@ -55,6 +55,9 @@ class Coder:
         openai.api_key = openai_api_key
         openai.api_base = openai_api_base
 
+        if not main_model:
+            main_model = models.GPT35_16k
+
         if not main_model.always_available:
             if not check_model_availability(main_model):
                 if main_model != models.GPT4: