diff --git a/src/azure-cli-core/azure/cli/core/parser.py b/src/azure-cli-core/azure/cli/core/parser.py
index 83d5261c120..fccd9cce814 100644
--- a/src/azure-cli-core/azure/cli/core/parser.py
+++ b/src/azure-cli-core/azure/cli/core/parser.py
@@ -359,7 +359,8 @@ def _search_in_extension_commands(self, command_str):
             ...
         }
         """
-
+        if not command_str:
+            return None
         cmd_chain = self._get_extension_command_tree()
         if not cmd_chain:
             return None