diff --git a/CHANGELOG.md b/CHANGELOG.md
index 08e271b2..088c8550 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -47,6 +47,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 
+- Fix type errors in PHP strict mode (#664)
 - Fix comment parsing to support multiple comments (#672)
 - Fix undefined local variable in `CalcFunction::parse()` (#593)
 - Fix PHP notice caused by parsing invalid color values having less than 6 characters (#485)
diff --git a/src/Value/Size.php b/src/Value/Size.php
index bf40f4a0..179e739f 100644
--- a/src/Value/Size.php
+++ b/src/Value/Size.php
@@ -204,7 +204,7 @@ public function render(OutputFormat $oOutputFormat): string
         $l = \localeconv();
         $sPoint = \preg_quote($l['decimal_point'], '/');
         $sSize = \preg_match('/[\\d\\.]+e[+-]?\\d+/i', (string) $this->fSize)
-            ? \preg_replace("/$sPoint?0+$/", '', \sprintf('%f', $this->fSize)) : $this->fSize;
+            ? \preg_replace("/$sPoint?0+$/", '', \sprintf('%f', $this->fSize)) : (string) $this->fSize;
         return \preg_replace(["/$sPoint/", '/^(-?)0\\./'], ['.', '$1.'], $sSize)
             . ($this->sUnit ?? '');
     }
diff --git a/tests/ParserTest.php b/tests/ParserTest.php
index 2262224f..e25c789d 100644
--- a/tests/ParserTest.php
+++ b/tests/ParserTest.php
@@ -278,7 +278,7 @@ public function specificity(): void
             new Selector('ol li::before', true),
         ], $oDoc->getSelectorsBySpecificity('< 100'));
         self::assertEquals([new Selector('li.green', true)], $oDoc->getSelectorsBySpecificity('11'));
-        self::assertEquals([new Selector('ol li::before', true)], $oDoc->getSelectorsBySpecificity(3));
+        self::assertEquals([new Selector('ol li::before', true)], $oDoc->getSelectorsBySpecificity('3'));
     }
 
     /**