diff --git a/src/dotenv/main.py b/src/dotenv/main.py
index 0c81bba5..822bba27 100644
--- a/src/dotenv/main.py
+++ b/src/dotenv/main.py
@@ -170,6 +170,10 @@ def set_key(
         quote_mode == "auto" and not value_to_set.isalnum()
     )
 
+    if not os.path.exists(dotenv_path):
+        logger.warning("Can't set key to %s - it doesn't exist.", dotenv_path)
+        return None, key_to_set, value_to_set
+
     if quote:
         value_out = "'{}'".format(value_to_set.replace("'", "\\'"))
     else: