diff --git a/docs/libcurl/opts/CURLOPT_DEBUGFUNCTION.3 b/docs/libcurl/opts/CURLOPT_DEBUGFUNCTION.3
index 9676f71dbc..c3a08f14f7 100644
--- a/docs/libcurl/opts/CURLOPT_DEBUGFUNCTION.3
+++ b/docs/libcurl/opts/CURLOPT_DEBUGFUNCTION.3
@@ -62,6 +62,7 @@ not null-terminated, but is exactly of the \fIsize\fP as told by the
 The \fIclientp\fP argument is the pointer set with \fICURLOPT_DEBUGDATA(3)\fP.
 
 Available \fBcurl_infotype\fP values:
+.RS
 .IP CURLINFO_TEXT
 The data is informational text.
 .IP CURLINFO_HEADER_IN
@@ -79,6 +80,13 @@ The data is protocol data sent to the peer.
 The data is SSL/TLS (binary) data sent to the peer.
 .IP CURLINFO_SSL_DATA_IN
 The data is SSL/TLS (binary) data received from the peer.
+.RE
+
+WARNING: This callback may be called with the curl \fIhandle\fP set to an
+internal handle. (Added in 8.4.0)
+
+If you need to distinguish your curl \fIhandle\fP from internal handles then
+set \fICURLOPT_PRIVATE(3)\fP on your handle.
 .SH DEFAULT
 NULL
 .SH PROTOCOLS
diff --git a/lib/curl_trc.c b/lib/curl_trc.c
index 76c35ba386..e53b30573f 100644
--- a/lib/curl_trc.c
+++ b/lib/curl_trc.c
@@ -61,6 +61,10 @@ void Curl_debug(struct Curl_easy *data, curl_infotype type,
       "* ", "< ", "> ", "{ ", "} ", "{ ", "} " };
     if(data->set.fdebug) {
       bool inCallback = Curl_is_in_callback(data);
+      /* CURLOPT_DEBUGFUNCTION doc says the user may set CURLOPT_PRIVATE to
+         distinguish their handle from internal handles. */
+      if(data->internal)
+        DEBUGASSERT(!data->set.private_data);
       Curl_set_in_callback(data, true);
       (void)(*data->set.fdebug)(data, type, ptr, size, data->set.debugdata);
       Curl_set_in_callback(data, inCallback);