diff --git a/.github/workflows/go-cross.yml b/.github/workflows/go-cross.yml
index bcfdc62a..30ec652a 100644
--- a/.github/workflows/go-cross.yml
+++ b/.github/workflows/go-cross.yml
@@ -16,7 +16,7 @@ jobs:
 
     strategy:
       matrix:
-        go-version: [ stable ]
+        go-version: [ oldstable, stable ]
         os: [ubuntu-latest, macos-latest, windows-latest]
 
     steps:
diff --git a/challenge/dns01/nameserver.go b/challenge/dns01/nameserver.go
index a2149b3f..a8d678af 100644
--- a/challenge/dns01/nameserver.go
+++ b/challenge/dns01/nameserver.go
@@ -48,7 +48,11 @@ func (cache *soaCacheEntry) isExpired() bool {
 
 // ClearFqdnCache clears the cache of fqdn to zone mappings. Primarily used in testing.
 func ClearFqdnCache() {
-	fqdnSoaCache.Clear()
+	// TODO(ldez): use `fqdnSoaCache.Clear()` when updating to go1.23
+	fqdnSoaCache.Range(func(k, v any) bool {
+		fqdnSoaCache.Delete(k)
+		return true
+	})
 }
 
 func AddDNSTimeout(timeout time.Duration) ChallengeOption {