diff --git a/src/AngleSharp.Css.Tests/Styling/CssSheet.cs b/src/AngleSharp.Css.Tests/Styling/CssSheet.cs
index 26a37a8..cf2d448 100644
--- a/src/AngleSharp.Css.Tests/Styling/CssSheet.cs
+++ b/src/AngleSharp.Css.Tests/Styling/CssSheet.cs
@@ -89,6 +89,76 @@ public void CssSheetSerializeBorder1pxSolidWithColor()
             Assert.AreEqual(expected, cssText);
         }
 
+        [Test]
+        public void CssSheetSerializeBorderSideThenBorderAll1()
+        {
+            var cssSrc = "#rule1 { border-top: none; border: 1px solid #BBCCEB }";
+            var expected = "#rule1 { border: 1px solid rgba(187, 204, 235, 1) }";
+            var stylesheet = ParseStyleSheet(cssSrc);
+            var cssText = stylesheet.ToCss();
+            Assert.AreEqual(expected, cssText);
+        }
+
+        [Test]
+        public void CssSheetSerializeBorderSideThenBorderAll2()
+        {
+            var cssSrc = "#rule1 { border-top: none !important; border: 1px solid #BBCCEB }";
+            var expected = "#rule1 { border-top: none !important; border-right: 1px solid rgba(187, 204, 235, 1); border-bottom: 1px solid rgba(187, 204, 235, 1); border-left: 1px solid rgba(187, 204, 235, 1) }";
+            var stylesheet = ParseStyleSheet(cssSrc);
+            var cssText = stylesheet.ToCss();
+            Assert.AreEqual(expected, cssText);
+        }
+
+        [Test]
+        public void CssSheetSerializeBorderSideThenBorderAll3()
+        {
+            var cssSrc = "#rule1 { border-top: none; border: 1px solid #BBCCEB !important }";
+            var expected = "#rule1 { border: 1px solid rgba(187, 204, 235, 1) !important }";
+            var stylesheet = ParseStyleSheet(cssSrc);
+            var cssText = stylesheet.ToCss();
+            Assert.AreEqual(expected, cssText);
+        }
+
+        [Test]
+        public void CssSheetSerializeBorderSideThenBorderAll4()
+        {
+            var cssSrc = "#rule1 { border-top: none !important; border: 1px solid #BBCCEB !important }";
+            var expected = "#rule1 { border: 1px solid rgba(187, 204, 235, 1) !important }";
+            var stylesheet = ParseStyleSheet(cssSrc);
+            var cssText = stylesheet.ToCss();
+            Assert.AreEqual(expected, cssText);
+        }
+
+        [Test]
+        public void CssSheetSerializeBorderAllThenBorderSide1()
+        {
+            var cssSrc = "#rule1 { border: 1px solid #BBCCEB !important; border-top: none }";
+            var expected = "#rule1 { border: 1px solid rgba(187, 204, 235, 1) !important }";
+            var stylesheet = ParseStyleSheet(cssSrc);
+            var cssText = stylesheet.ToCss();
+            Assert.AreEqual(expected, cssText);
+        }
+
+        [Test]
+        public void CssSheetSerializeBorderAllThenBorderSide2()
+        {
+            var cssSrc = "#rule1 { border: 1px solid #BBCCEB; border-top: none !important }";
+            var expected = "#rule1 { border-top: none !important; border-right: 1px solid rgba(187, 204, 235, 1); border-bottom: 1px solid rgba(187, 204, 235, 1); border-left: 1px solid rgba(187, 204, 235, 1) }";
+            var stylesheet = ParseStyleSheet(cssSrc);
+            var cssText = stylesheet.ToCss();
+            Assert.AreEqual(expected, cssText);
+        }
+
+        [Test]
+        public void CssSheetSerializeBorderAllThenBorderSide3()
+        {
+            var cssSrc = "#rule1 { border: 1px solid #BBCCEB !important; border-top: none !important }";
+            var expected = "#rule1 { border-top: none !important; border-right: 1px solid rgba(187, 204, 235, 1) !important; border-bottom: 1px solid rgba(187, 204, 235, 1) !important; border-left: 1px solid rgba(187, 204, 235, 1) !important }";
+            var stylesheet = ParseStyleSheet(cssSrc);
+            var cssText = stylesheet.ToCss();
+            Assert.AreEqual(expected, cssText);
+        }
+
         [Test]
         public void CssSheetSerializeBackgroundWithUrlPositionRepeatX()
         {
diff --git a/src/AngleSharp.Css/Values/Multiples/CssPeriodicValue.cs b/src/AngleSharp.Css/Values/Multiples/CssPeriodicValue.cs
index debfc48..f828861 100644
--- a/src/AngleSharp.Css/Values/Multiples/CssPeriodicValue.cs
+++ b/src/AngleSharp.Css/Values/Multiples/CssPeriodicValue.cs
@@ -61,7 +61,14 @@ public String CssText
 
                 for (var i = 0; i < l; i++)
                 {
-                    parts[i] = _values[i].CssText;
+                    var value = _values[i];
+
+                    if (value is null)
+                    {
+                        return String.Empty;
+                    }
+
+                    parts[i] = value.CssText;
                 }
 
                 if (l == 4 && parts[3].Is(parts[1]))