diff --git a/DB.php b/DB.php
index ea456e1..7227f99 100644
--- a/DB.php
+++ b/DB.php
@@ -591,7 +591,7 @@ function apiVersion()
      */
     function isError($value)
     {
-        return is_a($value, 'DB_Error');
+        return is_object($value) && is_a($value, 'DB_Error');		
     }
 
     // }}}
diff --git a/DB/common.php b/DB/common.php
index ae88fee..ac043f3 100644
--- a/DB/common.php
+++ b/DB/common.php
@@ -1249,7 +1249,7 @@ function &limitQuery($query, $from, $count, $params = array())
             return $query;
         }
         $result = $this->query($query, $params);
-        if (is_a($result, 'DB_result')) {
+        if (is_object($result) && is_a($result, 'DB_result')) {
             $result->setOption('limit_from', $from);
             $result->setOption('limit_count', $count);
         }
diff --git a/DB/mysqli.php b/DB/mysqli.php
index 52cf8f2..0eb850c 100644
--- a/DB/mysqli.php
+++ b/DB/mysqli.php
@@ -993,7 +993,7 @@ function tableInfo($result, $mode = null)
             $got_string = false;
         }
 
-        if (!is_a($id, 'mysqli_result')) {
+        if (!is_object($id) || !is_a($id, 'mysqli_result')) {
             return $this->mysqliRaiseError(DB_ERROR_NEED_MORE_DATA);
         }