diff --git a/src/PhpseclibV2/SftpConnectionProvider.php b/src/PhpseclibV2/SftpConnectionProvider.php
index efd8ab5d1..c04a12ede 100644
--- a/src/PhpseclibV2/SftpConnectionProvider.php
+++ b/src/PhpseclibV2/SftpConnectionProvider.php
@@ -143,7 +143,12 @@ private function checkFingerprint(SFTP $connection): void
             return;
         }
 
-        $publicKey = $connection->getServerPublicHostKey() ?: 'no-public-key';
+        $publicKey = $connection->getServerPublicHostKey();
+
+        if ($publicKey === false) {
+            throw UnableToEstablishAuthenticityOfHost::becauseTheAuthenticityCantBeEstablished($this->host);
+        }
+
         $fingerprint = $this->getFingerprintFromPublicKey($publicKey);
 
         if (0 !== strcasecmp($this->hostFingerprint, $fingerprint)) {
diff --git a/src/PhpseclibV3/SftpConnectionProvider.php b/src/PhpseclibV3/SftpConnectionProvider.php
index 4dda92093..082e6d3fb 100644
--- a/src/PhpseclibV3/SftpConnectionProvider.php
+++ b/src/PhpseclibV3/SftpConnectionProvider.php
@@ -150,7 +150,12 @@ private function checkFingerprint(SFTP $connection): void
             return;
         }
 
-        $publicKey = $connection->getServerPublicHostKey() ?: 'no-public-key';
+        $publicKey = $connection->getServerPublicHostKey();
+
+        if ($publicKey === false) {
+            throw UnableToEstablishAuthenticityOfHost::becauseTheAuthenticityCantBeEstablished($this->host);
+        }
+
         $fingerprint = $this->getFingerprintFromPublicKey($publicKey);
 
         if (0 !== strcasecmp($this->hostFingerprint, $fingerprint)) {