diff --git a/src/AngleSharp.Css/Dom/Internal/CssStyleDeclaration.cs b/src/AngleSharp.Css/Dom/Internal/CssStyleDeclaration.cs
index 42ecbe4..11f937e 100644
--- a/src/AngleSharp.Css/Dom/Internal/CssStyleDeclaration.cs
+++ b/src/AngleSharp.Css/Dom/Internal/CssStyleDeclaration.cs
@@ -377,6 +377,11 @@ private void RemovePropertyByName(String propertyName)
 
         private void ChangeDeclarations(IEnumerable<ICssProperty> decls, Predicate<ICssProperty> defaultSkip, Func<ICssProperty, ICssProperty, Boolean> removeExisting)
         {
+            if (decls is null)
+            {
+                return;
+            }
+
             var declarations = new List<ICssProperty>();
 
             foreach (var newdecl in decls)