diff --git a/aider/coder.py b/aider/coder.py
index 1890d2d4a..45e72b8e4 100755
--- a/aider/coder.py
+++ b/aider/coder.py
@@ -20,6 +20,9 @@ from aider import prompts
 from aider.commands import Commands
 from aider.repomap import RepoMap
 
+class MissingAPIKeyError(ValueError):
+    pass
+
 class Coder:
     abs_fnames = None
     repo = None
@@ -43,7 +46,8 @@ class Coder:
         if openai_api_key:
             openai.api_key = openai_api_key
         else:
-            openai.api_key = os.getenv("OPENAI_API_KEY")
+            raise MissingAPIKeyError('No OpenAI API key provided.')
+
         self.verbose = verbose
         self.abs_fnames = set()
         self.cur_messages = []
diff --git a/aider/main.py b/aider/main.py
index ba91e86eb..dd0d87734 100644
--- a/aider/main.py
+++ b/aider/main.py
@@ -131,6 +131,7 @@ def main(args=None, input=None, output=None):
         "--openai-api-key",
         metavar="OPENAI_API_KEY",
         help="Specify the OpenAI API key",
+        env_var="OPENAI_API_KEY",
     )
     parser.add_argument(
         "--dry-run",