diff --git a/lib/capsule.c b/lib/capsule.c
index f8dfcc050c..2d9af5cea5 100644
--- a/lib/capsule.c
+++ b/lib/capsule.c
@@ -135,8 +135,15 @@ static CURLcode capsule_decode_varint_at(struct bufq *recvbufq,
   return CURLE_OK;
 }
 
-size_t Curl_capsule_encap_udp_hdr(uint8_t *hdr, size_t hdrlen,
-                                  size_t payload_len)
+/**
+ * Write the capsule header (type + varint length + context ID) into `hdr`.
+ * @param hdr         Output buffer (must be >= HTTP_CAPSULE_HEADER_MAX_SIZE)
+ * @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
+ */
+static 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);
@@ -156,7 +163,7 @@ CURLcode Curl_capsule_encap_udp_datagram(struct dynbuf *dyn,
   size_t hdr_len;
 
   curlx_dyn_init(dyn, HTTP_CAPSULE_HEADER_MAX_SIZE + blen);
-  hdr_len = Curl_capsule_encap_udp_hdr(hdr, sizeof(hdr), blen);
+  hdr_len = capsule_encap_udp_hdr(hdr, sizeof(hdr), blen);
   DEBUGASSERT(hdr_len);
   if(!hdr_len)
     return CURLE_FAILED_INIT;
diff --git a/lib/capsule.h b/lib/capsule.h
index fa7dec19cb..4d50f78360 100644
--- a/lib/capsule.h
+++ b/lib/capsule.h
@@ -36,16 +36,6 @@
 
 /* HTTP Capsule function prototypes */
 
-/**
- * Write the capsule header (type + varint length + context ID) into `hdr`.
- * @param hdr         Output buffer (must be >= HTTP_CAPSULE_HEADER_MAX_SIZE)
- * @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
- */
-size_t Curl_capsule_encap_udp_hdr(uint8_t *hdr, size_t hdrlen,
-                                  size_t payload_len);
-
 /**
  * Encapsulate UDP payload into HTTP Datagram capsule format
  * @param dyn   Dynamic buffer to write capsule to