diff --git a/src/TwiGrid/Components/Translator.php b/src/TwiGrid/Components/Translator.php
index 69e09a0..a8ed0e3 100644
--- a/src/TwiGrid/Components/Translator.php
+++ b/src/TwiGrid/Components/Translator.php
@@ -49,7 +49,10 @@ public function addDictionary(array $dictionary): self
 	}
 
 
-	/** @param  string ...$parameters */
+	/**
+	 * @param  string $message
+	 * @param  string ...$parameters
+	 */
 	public function translate($message, ...$parameters): string
 	{
 		assert(is_string($message));
diff --git a/src/TwiGrid/DataGrid.php b/src/TwiGrid/DataGrid.php
index 0534fcf..fa8997b 100644
--- a/src/TwiGrid/DataGrid.php
+++ b/src/TwiGrid/DataGrid.php
@@ -12,6 +12,7 @@
 
 namespace TwiGrid;
 
+use Nette\Utils\Arrays;
 use Nette\Application\UI\Link;
 use Nette\Http\SessionSection;
 use TwiGrid\Components\Action;
@@ -293,7 +294,7 @@ public function setTranslator(ITranslator $translator): self
 
 	public function translate(string $s, int $count = null): string
 	{
-		return $this->getTranslator()->translate($s, $count);
+		return (string) $this->getTranslator()->translate($s, $count);
 	}
 
 
@@ -502,7 +503,20 @@ private function getRecordHandler(): RecordHandler
 	/** @param  string|string[] $primaryKey */
 	public function setPrimaryKey($primaryKey): self
 	{
-		$this->getRecordHandler()->setPrimaryKey(is_array($primaryKey) ? $primaryKey : func_get_args());
+		if (is_array($primaryKey)) {
+			$keys = $primaryKey;
+
+		} else {
+			/** @var string[] $keys */
+			$keys = func_get_args();
+
+			assert(Arrays::every($keys, function ($s): bool {
+				return is_string($s);
+
+			}), 'Primary keys must be an array of strings.');
+		}
+
+		$this->getRecordHandler()->setPrimaryKey($keys);
 		return $this;
 	}