diff --git a/phpBB/includes/functions.php b/phpBB/includes/functions.php
index 33545ab845..8688ba3f7e 100644
--- a/phpBB/includes/functions.php
+++ b/phpBB/includes/functions.php
@@ -289,7 +289,7 @@ function phpbb_gmgetdate($time = false)
 /**
 * Return formatted string for filesizes
 *
-* @param int	$value			filesize in bytes
+* @param int	$value			filesize in bytes (non-negative number)
 * @param bool	$string_only	true if language string should be returned
 * @param array	$allowed_units	only allow these units (data array indexes)
 *
diff --git a/tests/functions/get_formatted_filesize_test.php b/tests/functions/get_formatted_filesize_test.php
index 88866f90ac..c4793e8073 100644
--- a/tests/functions/get_formatted_filesize_test.php
+++ b/tests/functions/get_formatted_filesize_test.php
@@ -30,18 +30,10 @@ class phpbb_get_formatted_filesize_test extends phpbb_test_case
 
 			array(0, '0 BYTES'),
 			array(2, '2 BYTES'),
-			array(-2, '-2 BYTES'),
 
 			array(1023, '1023 BYTES'),
 			array(1025, '1 KIB'),
-			array(-1023, '-1023 BYTES'),
-			array(-1025, '-1025 BYTES'),
-
 			array(1048575, '1024 KIB'),
-
-			// large negatives
-			array(-1073741824, '-1073741824 BYTES'),
-			array(-1099511627776, '-1099511627776 BYTES'),
 		);
 	}
 
@@ -64,18 +56,10 @@ class phpbb_get_formatted_filesize_test extends phpbb_test_case
 
 			array('0', '0 BYTES'),
 			array('2', '2 BYTES'),
-			array('-2', '-2 BYTES'),
 
 			array('1023', '1023 BYTES'),
 			array('1025', '1 KIB'),
-			array('-1023', '-1023 BYTES'),
-			array('-1025', '-1025 BYTES'),
-
 			array('1048575', '1024 KIB'),
-
-			// large negatives
-			array('-1073741824', '-1073741824 BYTES'),
-			array('-1099511627776', '-1099511627776 BYTES'),
 		);
 	}