diff --git a/bids-validator/src/schema/expressionLanguage.ts b/bids-validator/src/schema/expressionLanguage.ts
index 9817e982a..88e58e5ce 100644
--- a/bids-validator/src/schema/expressionLanguage.ts
+++ b/bids-validator/src/schema/expressionLanguage.ts
@@ -1,4 +1,8 @@
 function exists(list: string[], rule: string = 'dataset'): number {
+  if (list == null) {
+    return 0
+  }
+
   const prefix: string[] = []
 
   // Stimuli and subject-relative paths get prefixes