diff --git a/lib/capsule.c b/lib/capsule.c
index 2d9af5cea5..4f5d0e7e24 100644
--- a/lib/capsule.c
+++ b/lib/capsule.c
@@ -141,9 +141,13 @@ static CURLcode capsule_decode_varint_at(struct bufq *recvbufq,
  * @param hdrlen      Size of `hdr` in bytes
  * @param payload_len Length of the UDP payload that follows
  * @return Number of header bytes written, or 0 on error
+ *
+ * @unittest 3400
  */
-static size_t capsule_encap_udp_hdr(uint8_t *hdr, size_t hdrlen,
-                                    size_t payload_len)
+UNITTEST size_t capsule_encap_udp_hdr(uint8_t *hdr, size_t hdrlen,
+                                      size_t payload_len);
+UNITTEST size_t capsule_encap_udp_hdr(uint8_t *hdr, size_t hdrlen,
+                                      size_t payload_len)
 {
   size_t off = 0;
   DEBUGASSERT(hdrlen >= HTTP_CAPSULE_HEADER_MAX_SIZE);
diff --git a/lib/capsule.h b/lib/capsule.h
index 4d50f78360..315ec8a61e 100644
--- a/lib/capsule.h
+++ b/lib/capsule.h
@@ -46,6 +46,9 @@
 CURLcode Curl_capsule_encap_udp_datagram(struct dynbuf *dyn,
                                          const void *buf, size_t blen);
 
+struct Curl_easy;
+struct Curl_cfilter;
+
 /**
  * Process one UDP capsule from buffer into raw datagram payload bytes.
  * @param cf        Connection filter
diff --git a/tests/unit/unit3400.c b/tests/unit/unit3400.c
index 2e083ddc5a..8df86c19f4 100644
--- a/tests/unit/unit3400.c
+++ b/tests/unit/unit3400.c
@@ -48,7 +48,7 @@ static void check_capsule_hdr(size_t payload_len,
   size_t hdr_len;
 
   memset(hdr, 0xA5, sizeof(hdr));
-  hdr_len = Curl_capsule_encap_udp_hdr(hdr, sizeof(hdr), payload_len);
+  hdr_len = capsule_encap_udp_hdr(hdr, sizeof(hdr), payload_len);
   fail_unless(hdr_len == expected_len, "capsule header length mismatch");
   fail_unless(!memcmp(hdr, expected, expected_len),
               "capsule header bytes mismatch");