diff --git a/lib/src/css_parser.dart b/lib/src/css_parser.dart
index 700c729a7c..a46a6d6ad6 100644
--- a/lib/src/css_parser.dart
+++ b/lib/src/css_parser.dart
@@ -370,7 +370,11 @@ class DeclarationVisitor extends css.Visitor {
 
   @override
   void visitExpressions(css.Expressions node) {
-    _properties[_currentProperty]!.addAll(node.expressions);
+    if (_properties[_currentProperty] != null) {
+      _properties[_currentProperty]!.addAll(node.expressions);
+    } else {
+      _properties[_currentProperty] = node.expressions;
+    }
   }
 }