diff --git a/src/render/expression.ts b/src/render/expression.ts
index d73eb357c0..9c2b6a8949 100644
--- a/src/render/expression.ts
+++ b/src/render/expression.ts
@@ -12,7 +12,7 @@ import { parseStringLiteral } from '../parser/parse-string-literal'
 import { Context } from '../context/context'
 import { range } from '../util/underscore'
 import { Operators } from '../render/operator'
-import { UndefinedVariableError, InternalUndefinedVariableError } from '../util/error'
+import { UndefinedVariableError } from '../util/error'
 
 export class Expression {
   private postfix: Token[]
@@ -52,7 +52,7 @@ function evalPropertyAccessToken (token: PropertyAccessToken, ctx: Context, leni
   try {
     return ctx.get([variable, ...props])
   } catch (e) {
-    if (lenient && e instanceof InternalUndefinedVariableError) return null
+    if (lenient && e.name === 'InternalUndefinedVariableError') return null
     throw (new UndefinedVariableError(e, token))
   }
 }
diff --git a/src/util/error.ts b/src/util/error.ts
index f7a96776d7..1caf82e7cf 100644
--- a/src/util/error.ts
+++ b/src/util/error.ts
@@ -44,7 +44,7 @@ export class RenderError extends LiquidError {
     super.update()
   }
   public static is (obj: any): obj is RenderError {
-    return obj instanceof RenderError
+    return obj.name === 'RenderError'
   }
 }
 
diff --git a/test/e2e/issues.ts b/test/e2e/issues.ts
index fddccbeff3..63ca145ab7 100644
--- a/test/e2e/issues.ts
+++ b/test/e2e/issues.ts
@@ -1,4 +1,5 @@
 import { Liquid } from '../..'
+import { Liquid as LiquidUMD } from '../../dist/liquid.browser.umd.js'
 import { expect, use } from 'chai'
 import * as chaiAsPromised from 'chai-as-promised'
 
@@ -78,4 +79,12 @@ describe('Issues', function () {
     )
     expect(html).to.equal('BAR')
   })
+  it('#313 lenientIf not working as expected in umd', async () => {
+    const engine = new LiquidUMD({
+      strictVariables: true,
+      lenientIf: true
+    })
+    const html = await engine.parseAndRender(`{{ name | default: "default name" }}`)
+    expect(html).to.equal('default name')
+  })
 })