diff --git a/lldb/packages/Python/lldbsuite/test/tools/lldb-dap/dap_server.py b/lldb/packages/Python/lldbsuite/test/tools/lldb-dap/dap_server.py
index 4a7ba78b63993..7a9d5a82983d7 100644
--- a/lldb/packages/Python/lldbsuite/test/tools/lldb-dap/dap_server.py
+++ b/lldb/packages/Python/lldbsuite/test/tools/lldb-dap/dap_server.py
@@ -391,7 +391,7 @@ def _process_recv_packets(self) -> None:
         with self._recv_condition:
             for packet in self._recv_packets:
                 if packet and ("seq" not in packet or packet["seq"] == 0):
-                    warnings.warn(
+                    raise ValueError(
                         f"received a malformed packet, expected 'seq != 0' for {packet!r}"
                     )
                 # Handle events that may modify any stateful properties of
diff --git a/lldb/tools/lldb-dap/DAP.cpp b/lldb/tools/lldb-dap/DAP.cpp
index 465d85a07bd34..6971bfea5c128 100644
--- a/lldb/tools/lldb-dap/DAP.cpp
+++ b/lldb/tools/lldb-dap/DAP.cpp
@@ -274,8 +274,11 @@ Id DAP::Send(const Message &message) {
   std::lock_guard<std::mutex> guard(call_mutex);
   Message msg = std::visit(
       [this](auto &&msg) -> Message {
-        if (msg.seq == kCalculateSeq)
-          msg.seq = seq++;
+        if (msg.seq == kCalculateSeq) {
+          seq++;
+          msg.seq = seq;
+        }
+        assert(msg.seq > 0 && "message sequence must be greater than zero.");
         return msg;
       },
       Message(message));
diff --git a/lldb/tools/lldb-dap/Protocol/ProtocolBase.h b/lldb/tools/lldb-dap/Protocol/ProtocolBase.h
index 42c6c8890af24..09ce6802b17c0 100644
--- a/lldb/tools/lldb-dap/Protocol/ProtocolBase.h
+++ b/lldb/tools/lldb-dap/Protocol/ProtocolBase.h
@@ -31,11 +31,11 @@ namespace lldb_dap::protocol {
 // MARK: Base Protocol
 
 /// Message unique identifier type.
-using Id = int64_t;
+using Id = uint64_t;
 
 /// A unique identifier that indicates the `seq` field should be calculated by
 /// the current session.
-static constexpr Id kCalculateSeq = INT64_MAX;
+static constexpr Id kCalculateSeq = UINT64_MAX;
 
 /// A client or debug adapter initiated request.
 struct Request {