diff --git a/lib/altsvc.c b/lib/altsvc.c
index bda3bf4e0a..7bca840151 100644
--- a/lib/altsvc.c
+++ b/lib/altsvc.c
@@ -462,6 +462,7 @@ CURLcode Curl_altsvc_parse(struct Curl_easy *data,
   struct altsvc *as;
   unsigned short dstport = srcport; /* the same by default */
   CURLcode result = getalnum(&p, alpnbuf, sizeof(alpnbuf));
+  size_t entries = 0;
 #ifdef CURL_DISABLE_VERBOSE_STRINGS
   (void)data;
 #endif
@@ -472,11 +473,10 @@ CURLcode Curl_altsvc_parse(struct Curl_easy *data,
 
   DEBUGASSERT(asi);
 
-  /* Flush all cached alternatives for this source origin, if any */
-  altsvc_flush(asi, srcalpnid, srchost, srcport);
-
   /* "clear" is a magic keyword */
   if(strcasecompare(alpnbuf, "clear")) {
+    /* Flush cached alternatives for this source origin */
+    altsvc_flush(asi, srcalpnid, srchost, srcport);
     return CURLE_OK;
   }
 
@@ -494,6 +494,7 @@ CURLcode Curl_altsvc_parse(struct Curl_easy *data,
         bool quoted = FALSE;
         time_t maxage = 24 * 3600; /* default is 24 hours */
         bool persist = FALSE;
+        bool valid = TRUE;
         p++;
         if(*p != ':') {
           /* host name starts here */
@@ -503,7 +504,7 @@ CURLcode Curl_altsvc_parse(struct Curl_easy *data,
           len = p - hostp;
           if(!len || (len >= MAX_ALTSVC_HOSTLEN)) {
             infof(data, "Excessive alt-svc host name, ignoring.");
-            dstalpnid = ALPN_none;
+            valid = FALSE;
           }
           else {
             memcpy(namebuf, hostp, len);
@@ -520,10 +521,11 @@ CURLcode Curl_altsvc_parse(struct Curl_easy *data,
           unsigned long port = strtoul(++p, &end_ptr, 10);
           if(port > USHRT_MAX || end_ptr == p || *end_ptr != '\"') {
             infof(data, "Unknown alt-svc port number, ignoring.");
-            dstalpnid = ALPN_none;
+            valid = FALSE;
           }
+          else
+            dstport = curlx_ultous(port);
           p = end_ptr;
-          dstport = curlx_ultous(port);
         }
         if(*p++ != '\"')
           break;
@@ -575,7 +577,12 @@ CURLcode Curl_altsvc_parse(struct Curl_easy *data,
               persist = TRUE;
           }
         }
-        if(dstalpnid) {
+        if(dstalpnid && valid) {
+          if(!entries++)
+            /* Flush cached alternatives for this source origin, if any - when
+               this is the first entry of the line. */
+            altsvc_flush(asi, srcalpnid, srchost, srcport);
+
           as = altsvc_createid(srchost, dsthost,
                                srcalpnid, dstalpnid,
                                srcport, dstport);
@@ -589,10 +596,6 @@ CURLcode Curl_altsvc_parse(struct Curl_easy *data,
                   Curl_alpnid2str(dstalpnid));
           }
         }
-        else {
-          infof(data, "Unknown alt-svc protocol \"%s\", skipping.",
-                alpnbuf);
-        }
       }
       else
         break;
diff --git a/tests/data/test356 b/tests/data/test356
index 0f87958997..9f6459487c 100644
--- a/tests/data/test356
+++ b/tests/data/test356
@@ -16,7 +16,9 @@ Content-Length: 6
 Connection: close
 Content-Type: text/html
 Funny-head: yesyes
+Alt-Svc: h1="nowhere.foo:-1"
 Alt-Svc: h1="nowhere.foo:81", un-kno22!wn=":82"
+Alt-Svc: h1="nowhere.foo:70000"
 
 -foo-
 </data>