diff --git a/scrapy/core/http2/agent.py b/scrapy/core/http2/agent.py
index f4ac29bc6..f829cc5f8 100644
--- a/scrapy/core/http2/agent.py
+++ b/scrapy/core/http2/agent.py
@@ -68,7 +68,7 @@ class H2ConnectionPool:
 
         # Now as we have established a proper HTTP/2 connection
         # we fire all the deferred's with the connection instance
-        pending_requests = self._pending_requests.pop(key)
+        pending_requests = self._pending_requests.pop(key, None)
         while pending_requests:
             d = pending_requests.popleft()
             d.callback(conn)
diff --git a/scrapy/core/http2/protocol.py b/scrapy/core/http2/protocol.py
index 7a3156541..c6f00423a 100644
--- a/scrapy/core/http2/protocol.py
+++ b/scrapy/core/http2/protocol.py
@@ -39,7 +39,7 @@ class InvalidNegotiatedProtocol(H2Error):
         self.negotiated_protocol = negotiated_protocol
 
     def __str__(self) -> str:
-        return f'InvalidHostname: Expected h2 as negotiated protocol, received {self.negotiated_protocol!r}'
+        return f'InvalidNegotiatedProtocol: Expected h2 as negotiated protocol, received {self.negotiated_protocol!r}'
 
 
 class RemoteTerminatedConnection(H2Error):
diff --git a/scrapy/core/http2/stream.py b/scrapy/core/http2/stream.py
index 33302421e..df7470e11 100644
--- a/scrapy/core/http2/stream.py
+++ b/scrapy/core/http2/stream.py
@@ -31,6 +31,9 @@ class InactiveStreamClosed(ConnectionClosed):
     def __init__(self, request: Request):
         self.request = request
 
+    def __str__(self) -> str:
+        return f'InactiveStreamClosed: Connection was closed without sending the request {self.request!r}'
+
 
 class InvalidHostname(H2Error):