diff --git a/nodriver/core/_contradict.py b/nodriver/core/_contradict.py
index e35434d..c770361 100644
--- a/nodriver/core/_contradict.py
+++ b/nodriver/core/_contradict.py
@@ -139,7 +139,7 @@ def _camel_to_snake(s):
     :param str s: string to be converted
     :return: (str) snake_case version of s
     """
-    s = s.replace("-", "").replace(".", "")
+    s = s.replace("-", "").replace(".", "").replace(" ", "_")
 
     return __RE_CAMEL_TO_SNAKE__.sub(r"_\1", s).lower()
 
diff --git a/nodriver/core/connection.py b/nodriver/core/connection.py
index dfc3fff..51606d0 100644
--- a/nodriver/core/connection.py
+++ b/nodriver/core/connection.py
@@ -551,8 +551,12 @@ async def listener_loop(self):
                 # response to our command
                 if message["id"] in self.connection.mapper:
                     # get the corresponding Transaction
-                    tx = self.connection.mapper[message["id"]]
+
+                    # pop to prevent memory leaks
+                    # thanks to zxsleebu
+                    tx = self.connection.mapper.pop(message["id"])
                     logger.debug("got answer for %s", tx)
+
                     # complete the transaction, which is a Future object
                     # and thus will return to anyone awaiting it.
                     tx(**message)