diff --git a/lib/src/template.dart b/lib/src/template.dart
index bec14a2..1db8898 100644
--- a/lib/src/template.dart
+++ b/lib/src/template.dart
@@ -53,6 +53,7 @@
 }
 
 _Node _parse(String source, bool lenient, String templateName) {
+  if (source == null) throw new ArgumentError.notNull('Template source');
   var tokens = _scan(source, lenient);
   var ast = _parseTokens(tokens, lenient, templateName);
   return ast;