diff --git a/src/datadog/curl.cpp b/src/datadog/curl.cpp
index 3f62445f..05fef196 100644
--- a/src/datadog/curl.cpp
+++ b/src/datadog/curl.cpp
@@ -208,7 +208,7 @@ class CurlImpl {
   };
 
   void run();
-  void handle_message(const CURLMsg &);
+  void handle_message(const CURLMsg &, std::unique_lock<std::mutex> &);
   CURLcode log_on_error(CURLcode result);
   CURLMcode log_on_error(CURLMcode result);
 
@@ -476,7 +476,7 @@ void CurlImpl::run() {
     // us to handle.  Handle any pending messages.
     while ((message = curl_.multi_info_read(multi_handle_,
                                             &num_messages_remaining))) {
-      handle_message(*message);
+      handle_message(*message, lock);
     }
 
     const int max_wait_milliseconds = 10 * 1000;
@@ -514,7 +514,8 @@ void CurlImpl::run() {
   curl_.global_cleanup();
 }
 
-void CurlImpl::handle_message(const CURLMsg &message) {
+void CurlImpl::handle_message(const CURLMsg &message,
+                              std::unique_lock<std::mutex> &lock) {
   if (message.msg != CURLMSG_DONE) {
     return;
   }
@@ -536,8 +537,10 @@ void CurlImpl::handle_message(const CURLMsg &message) {
     error_message += curl_.easy_strerror(result);
     error_message += "): ";
     error_message += request.error_buffer;
+    lock.unlock();
     request.on_error(
         Error{Error::CURL_REQUEST_FAILURE, std::move(error_message)});
+    lock.lock();
   } else {
     long status;
     if (log_on_error(curl_.easy_getinfo_response_code(request_handle,
@@ -545,8 +548,10 @@ void CurlImpl::handle_message(const CURLMsg &message) {
       status = -1;
     }
     HeaderReader reader(&request.response_headers_lower);
+    lock.unlock();
     request.on_response(static_cast<int>(status), reader,
                         std::move(request.response_body));
+    lock.lock();
   }
 
   log_on_error(curl_.multi_remove_handle(multi_handle_, request_handle));
diff --git a/src/datadog/http_client.h b/src/datadog/http_client.h
index df340d43..2a7310ac 100644
--- a/src/datadog/http_client.h
+++ b/src/datadog/http_client.h
@@ -43,7 +43,8 @@ class HTTPClient {
   // a response is delivered (even if that response contains an error HTTP
   // response status).  Invoke the specified `on_error` if an error occurs
   // outside of HTTP, such as a connection failure.  If an error occurs while
-  // preparing the request, return an `Error`.
+  // preparing the request, return an `Error`. The behavior is undefined if
+  // either of `on_response` or `on_error` throws an exception.
   virtual Expected<void> post(const URL& url, HeadersSetter set_headers,
                               std::string body, ResponseHandler on_response,
                               ErrorHandler on_error) = 0;
diff --git a/test/test_tracer.cpp b/test/test_tracer.cpp
index 11c3ca8c..9947ddbf 100644
--- a/test/test_tracer.cpp
+++ b/test/test_tracer.cpp
@@ -1017,7 +1017,7 @@ TEST_CASE("128-bit trace IDs") {
   // Use a clock that always returns a hard-coded `TimePoint`.
   // May 6, 2010 14:45:13 America/New_York
   const std::time_t flash_crash = 1273171513;
-  const Clock clock = [flash_crash]() {
+  const Clock clock = []() {
     TimePoint result;
     result.wall = std::chrono::system_clock::from_time_t(flash_crash);
     return result;
diff --git a/test/test_tracer_telemetry.cpp b/test/test_tracer_telemetry.cpp
index 430a1860..5b92b88b 100644
--- a/test/test_tracer_telemetry.cpp
+++ b/test/test_tracer_telemetry.cpp
@@ -14,7 +14,7 @@ using namespace datadog::tracing;
 
 TEST_CASE("Tracer telemetry") {
   const std::time_t mock_time = 1672484400;
-  const Clock clock = [mock_time]() {
+  const Clock clock = []() {
     TimePoint result;
     result.wall = std::chrono::system_clock::from_time_t(mock_time);
     return result;