diff --git a/bids-validator/src/schema/expressionLanguage.ts b/bids-validator/src/schema/expressionLanguage.ts
index e6036a3f8..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
@@ -47,16 +51,20 @@ export const expressionFunctions = {
     if (Array.isArray(operand)) {
       return 'array'
     }
-    if (typeof operand === 'undefined') {
+    if (typeof operand === 'undefined' || operand === null) {
       return 'null'
     }
     return typeof operand
   },
-  min: (list: number[]): number => {
-    return Math.min(...list)
+  min: (list: number[]): number | null => {
+    return list != null
+      ? Math.min(...list.filter((x) => typeof x === 'number'))
+      : null
   },
-  max: (list: number[]): number => {
-    return Math.max(...list)
+  max: (list: number[]): number | null => {
+    return list != null
+      ? Math.max(...list.filter((x) => typeof x === 'number'))
+      : null
   },
   length: <T>(list: T[]): number | null => {
     if (Array.isArray(list) || typeof list == 'string') {
@@ -68,7 +76,10 @@ export const expressionFunctions = {
     return list.filter((x) => x === val).length
   },
   exists: exists,
-  substr: (arg: string, start: number, end: number): string => {
+  substr: (arg: string, start: number, end: number): string | null => {
+    if (arg == null || start == null || end == null) {
+      return null
+    }
     return arg.substr(start, end - start)
   },
   sorted: <T>(list: T[]): T[] => {
diff --git a/bids-validator/src/tests/schema-expression-language.test.ts b/bids-validator/src/tests/schema-expression-language.test.ts
index cc9b50e0c..798f640bf 100644
--- a/bids-validator/src/tests/schema-expression-language.test.ts
+++ b/bids-validator/src/tests/schema-expression-language.test.ts
@@ -4,10 +4,18 @@ import { colors } from '../deps/fmt.ts'
 import { BIDSContext } from '../schema/context.ts'
 import { assert, assertEquals } from '../deps/asserts.ts'
 import { evalCheck } from '../schema/applyRules.ts'
+import { expressionFunctions } from '../schema/expressionLanguage.ts'
 
 const schema = await loadSchema()
 const pretty_null = (x: string | null): string => (x === null ? 'null' : x)
 
+const equal = <T>(a: T, b: T): boolean => {
+  if (Array.isArray(a) && Array.isArray(b)) {
+    return a.length === b.length && a.every((val, idx) => val === b[idx])
+  }
+  return a === b
+}
+
 Deno.test('validate schema expression tests', async (t) => {
   const results: string[][] = []
   const header = ['expression', 'desired', 'actual', 'result'].map((x) =>
@@ -15,8 +23,10 @@ Deno.test('validate schema expression tests', async (t) => {
   )
   for (const test of schema.meta.expression_tests) {
     await t.step(`${test.expression} evals to ${test.result}`, () => {
-      const actual_result = evalCheck(test.expression, {} as BIDSContext)
-      if (actual_result == test.result) {
+      // @ts-expect-error
+      const context = expressionFunctions as BIDSContext
+      const actual_result = evalCheck(test.expression, context)
+      if (equal(actual_result, test.result)) {
         results.push([
           colors.cyan(test.expression),
           pretty_null(test.result),