diff --git a/pyprql/cli/cli.py b/pyprql/cli/cli.py
index b441804..2f94859 100644
--- a/pyprql/cli/cli.py
+++ b/pyprql/cli/cli.py
@@ -424,5 +424,5 @@ def run(self) -> None:
                 self.prompt_text = "PRQL> "
                 self.has_one_blank = False
 
-    def clean_input(self, command):
+    def clean_input(self, command: str) -> str:
         return re.sub(r"\|+", r"|", command.rstrip("|"))