diff --git a/tests/MD_STD_IN_Test.php b/tests/MD_STD_IN_Test.php
index 93bd4b5..ec400b6 100644
--- a/tests/MD_STD_IN_Test.php
+++ b/tests/MD_STD_IN_Test.php
@@ -116,7 +116,7 @@ final class MD_STD_IN_Test extends TestCase {
     /**
      * Data provider for invalid latitudes.
      *
-     * @return array<array{0: mixed, 1: string}>
+     * @return array<array{0: mixed, 1: class-string<Throwable>}>
      */
     public static function invalid_latitude_provider():array {
 
@@ -588,8 +588,8 @@ final class MD_STD_IN_Test extends TestCase {
     /**
      * Function for testing validate_longitude().
      *
-     * @param mixed  $to_validate    Input to validate.
-     * @param string $exceptionClass Exception class.
+     * @param mixed                   $to_validate    Input to validate.
+     * @param class-string<Throwable> $exceptionClass Exception class.
      *
      * @return void
      */
@@ -619,8 +619,8 @@ final class MD_STD_IN_Test extends TestCase {
     /**
      * Function for testing validate_latitude().
      *
-     * @param mixed  $to_validate    Input to validate.
-     * @param string $exceptionClass Exception class.
+     * @param mixed                   $to_validate    Input to validate.
+     * @param class-string<Throwable> $exceptionClass Exception class.
      *
      * @return void
      */
@@ -677,9 +677,16 @@ final class MD_STD_IN_Test extends TestCase {
      */
     public function test_ensureStringIsUtf8():void {
 
+        if (empty($convToIso8859 = iconv("UTF-8", 'ISO-8859-1//TRANSLIT', "ä"))) {
+            throw new Exception("Iconv returned empty result");
+        }
+        if (empty($convToIso2022 = iconv("UTF-8", 'ISO-2022-JP//TRANSLIT', "ä"))) {
+            throw new Exception("Iconv returned empty result");
+        }
+
         self::assertEquals("ä", MD_STD_IN::ensureStringIsUtf8("ä"));
-        self::assertEquals("ä", MD_STD_IN::ensureStringIsUtf8(iconv("UTF-8", 'ISO-8859-1//TRANSLIT', "ä")));
-        self::assertEquals("a", MD_STD_IN::ensureStringIsUtf8(iconv("UTF-8", 'ISO-2022-JP//TRANSLIT', "ä")));
+        self::assertEquals("ä", MD_STD_IN::ensureStringIsUtf8($convToIso8859));
+        self::assertEquals("a", MD_STD_IN::ensureStringIsUtf8($convToIso2022));
 
     }
 }
diff --git a/tests/MD_STD_Test.php b/tests/MD_STD_Test.php
index c931ccd..7aa0435 100644
--- a/tests/MD_STD_Test.php
+++ b/tests/MD_STD_Test.php
@@ -21,7 +21,7 @@ final class MD_STD_Test extends TestCase {
     /**
      * Data provider for returning a tmp file.
      *
-     * @return array<string, array{0: string, 1: string}>
+     * @return array<string, array{0: non-empty-string}>
      */
     public static function file_name_in_tmp_provider():array {
 
@@ -37,7 +37,7 @@ final class MD_STD_Test extends TestCase {
     /**
      * Data provider for invalid directories.
      *
-     * @return array<string, array{0: string}>
+     * @return array<non-empty-string, array{0: non-empty-string}>
      */
     public static function invalid_directory_provider():array {
 
@@ -118,7 +118,7 @@ final class MD_STD_Test extends TestCase {
     /**
      * Checks if a file can be written.
      *
-     * @param string $temp_file Tmp file.
+     * @param non-empty-string $temp_file Tmp file.
      *
      * @return void
      */
@@ -146,8 +146,8 @@ final class MD_STD_Test extends TestCase {
     /**
      * Check MD_STD::realpath throws exception on non-existing path.
      *
-     * @param string $to_validate    Input to validate.
-     * @param string $exceptionClass Exception class.
+     * @param non-empty-string        $to_validate    Input to validate.
+     * @param class-string<Throwable> $exceptionClass Exception class.
      *
      * @return void
      */
@@ -162,8 +162,8 @@ final class MD_STD_Test extends TestCase {
     /**
      * Checks if a file can be read.
      *
-     * @param string $to_validate    Input to validate.
-     * @param string $exceptionClass Exception class.
+     * @param non-empty-string        $to_validate    Input to validate.
+     * @param class-string<Throwable> $exceptionClass Exception class.
      *
      * @return void
      */
@@ -229,7 +229,7 @@ final class MD_STD_Test extends TestCase {
     /**
      * Checks if nothing happens if a file does not exist.
      *
-     * @param string $temp_file Tmp file.
+     * @param non-empty-string $temp_file Tmp file.
      *
      * @return void
      */
@@ -248,7 +248,7 @@ final class MD_STD_Test extends TestCase {
     /**
      * Checks unlink_if_exists works.
      *
-     * @param string $temp_file Tmp file.
+     * @param non-empty-string $temp_file Tmp file.
      *
      * @return void
      */
@@ -268,7 +268,10 @@ final class MD_STD_Test extends TestCase {
      */
     public function test_scandir():void {
 
-        $files = scandir(__DIR__);
+        if (empty($files = scandir(__DIR__))) {
+            throw new Exception("Running regular scandir() failed on script directory: " . __DIR__);
+        }
+
         $filesChecked = MD_STD::scandir(__DIR__);
 
         foreach ($filesChecked as $file) {
@@ -284,7 +287,7 @@ final class MD_STD_Test extends TestCase {
     /**
      * Checks invalid directory.
      *
-     * @param string $dir Dir name.
+     * @param non-empty-string $dir Dir name.
      *
      * @return void
      */
@@ -520,9 +523,9 @@ final class MD_STD_Test extends TestCase {
     /**
      * Checks that minimizeHTMLString works.
      *
-     * @param string        $filepath   File path.
-     * @param array<string> $mime_types Mime types expected.
-     * @param string        $exception  Exception class name.
+     * @param non-empty-string        $filepath   File path.
+     * @param array<string>           $mime_types Mime types expected.
+     * @param class-string<Throwable> $exception  Exception class.
      *
      * @return void
      */
@@ -568,7 +571,7 @@ final class MD_STD_Test extends TestCase {
     /**
      * Checks check_is_writable does not work with non-existent or non-directory paths.
      *
-     * @param string $dir Dir name.
+     * @param non-empty-string $dir Dir name.
      *
      * @return void
      */