diff --git a/src/util/BitField.js b/src/util/BitField.js
index 757da7ab9..f849e7b8a 100644
--- a/src/util/BitField.js
+++ b/src/util/BitField.js
@@ -146,7 +146,7 @@ class BitField {
     if (typeof bit === 'number' && bit >= 0) return bit;
     if (bit instanceof BitField) return bit.bitfield;
     if (Array.isArray(bit)) return bit.map(p => this.resolve(p)).reduce((prev, p) => prev | p, 0);
-    if (typeof bit === 'string') return this.FLAGS[bit];
+    if (typeof bit === 'string' && typeof this.FLAGS[bit] !== 'undefined') return this.FLAGS[bit];
     throw new RangeError('BITFIELD_INVALID');
   }
 }