diff --git a/src/AbstractCsv.php b/src/AbstractCsv.php
index 2c353a52..bd075373 100644
--- a/src/AbstractCsv.php
+++ b/src/AbstractCsv.php
@@ -200,7 +200,7 @@ public static function createFromPath($path, $open_mode = 'r+')
      */
     protected static function validateString($str)
     {
-        if (is_scalar($str) || (is_object($str) && method_exists($str, '__toString'))) {
+        if (is_string($str) || (is_object($str) && method_exists($str, '__toString'))) {
             return (string) $str;
         }
 
diff --git a/src/Reader.php b/src/Reader.php
index 9da44a61..e422d312 100644
--- a/src/Reader.php
+++ b/src/Reader.php
@@ -207,9 +207,7 @@ public function fetchAssoc($offset_or_keys = 0, callable $callable = null)
     protected function getAssocKeys($offset_or_keys)
     {
         if (is_array($offset_or_keys)) {
-            $this->assertValidAssocKeys($offset_or_keys);
-
-            return $offset_or_keys;
+            return $this->validateAssocKeys($offset_or_keys);
         }
 
         if (false === filter_var($offset_or_keys, FILTER_VALIDATE_INT, ['options' => ['min_range' => 0]])) {
@@ -217,7 +215,7 @@ protected function getAssocKeys($offset_or_keys)
         }
 
         $keys = $this->getRow($offset_or_keys);
-        $this->assertValidAssocKeys($keys);
+        $keys = $this->validateAssocKeys($keys);
         $filterOutRow = function ($row, $rowIndex) use ($offset_or_keys) {
             return is_array($row) && $rowIndex != $offset_or_keys;
         };
@@ -233,23 +231,22 @@ protected function getAssocKeys($offset_or_keys)
      *
      * @throws InvalidArgumentException If the submitted array fails the assertion
      */
-    protected function assertValidAssocKeys(array $keys)
+    protected function validateAssocKeys(array $keys)
     {
-        if (empty($keys) || $keys !== array_unique(array_filter($keys, [$this, 'isValidString']))) {
-            throw new InvalidArgumentException('Use a flat array with unique string values');
+        if (empty($keys)) {
+            throw new InvalidArgumentException('The array can not be empty');
         }
-    }
 
-    /**
-     * Returns whether the submitted value can be used as string
-     *
-     * @param mixed $value
-     *
-     * @return bool
-     */
-    protected function isValidString($value)
-    {
-        return is_scalar($value) || (is_object($value) && method_exists($value, '__toString'));
+        foreach ($keys as &$str) {
+            $str = $this->validateString($str);
+        }
+        unset($str);
+
+        if ($keys == array_unique($keys)) {
+            return $keys;
+        }
+
+        throw new InvalidArgumentException('The array must contain unique values');
     }
 
     /**
diff --git a/test/ReaderTest.php b/test/ReaderTest.php
index e16610c5..d650c510 100644
--- a/test/ReaderTest.php
+++ b/test/ReaderTest.php
@@ -215,6 +215,15 @@ public function testFetchAssocWithRowIndex()
         $this->assertContains(['D' => '6', 'E' => '7', 'F' => '8'], $res);
     }
 
+    /**
+     * @expectedException \InvalidArgumentException
+     */
+    public function testFetchAssocThrowsExceptionWithNonUniqueAssocKeys()
+    {
+        $keys = ['firstname', 'lastname', 'firstname'];
+        $this->csv->fetchAssoc($keys);
+    }
+
     /**
      * @param  $expected
      * @dataProvider validBOMSequences