diff --git a/src/Application/Model/Credential/PublicKeyCredential.php b/src/Application/Model/Credential/PublicKeyCredential.php
index e97a649..d688ed3 100755
--- a/src/Application/Model/Credential/PublicKeyCredential.php
+++ b/src/Application/Model/Credential/PublicKeyCredential.php
@@ -175,6 +175,6 @@ class PublicKeyCredential extends BaseModel
             );
         $oxid = $qb->execute()->fetchOne();
 
-        return strlen($oxid) ? $oxid : null;
+        return is_string($oxid) && strlen($oxid) ? $oxid : null;
     }
 }
\ No newline at end of file
diff --git a/src/Application/Model/Credential/PublicKeyCredentialList.php b/src/Application/Model/Credential/PublicKeyCredentialList.php
index 60af845..9267baa 100755
--- a/src/Application/Model/Credential/PublicKeyCredentialList.php
+++ b/src/Application/Model/Credential/PublicKeyCredentialList.php
@@ -66,7 +66,7 @@ class PublicKeyCredentialList extends ListModel implements PublicKeyCredentialSo
             );
         $credential = $qb->execute()->fetchOne();
 
-        if (!strlen($credential)) {
+        if (!is_string($credential) || !strlen($credential)) {
             return null;
         }