diff --git a/modules/ssl/ssl_util.c b/modules/ssl/ssl_util.c
index 12ffff511e2..9dc5ddb2b8d 100644
--- a/modules/ssl/ssl_util.c
+++ b/modules/ssl/ssl_util.c
@@ -201,9 +201,15 @@ ssl_asn1_t *ssl_asn1_table_set(apr_hash_t *table, const char *key,
 {
     apr_ssize_t klen = strlen(key);
     ssl_asn1_t *asn1 = apr_hash_get(table, key, klen);
-    apr_size_t length = i2d_PrivateKey(pkey, NULL);
+    int derlen = i2d_PrivateKey(pkey, NULL);
+    apr_size_t length;
     unsigned char *p;
 
+    /* Encoding the key length can only fail in pathological cases which
+     * cannot occur for a key which has already been loaded and used. */
+    ap_assert(derlen > 0);
+    length = (apr_size_t)derlen;
+
     /* Re-use structure if cached previously. */
     if (asn1) {
         if (asn1->nData != length) {