diff --git a/src/abstract/montgomery.ts b/src/abstract/montgomery.ts
index 69a6c8e..e8abc00 100644
--- a/src/abstract/montgomery.ts
+++ b/src/abstract/montgomery.ts
@@ -150,7 +150,8 @@ export function montgomery(curveDef: CurveType): CurveFn {
     // This is very ugly way, but it works because fieldLen-1 is outside of bounds for X448, so this becomes NOOP
     // fieldLen - scalaryBytes = 1 for X448 and = 0 for X25519
     const u = ensureBytes(uEnc, montgomeryBytes);
-    u[fieldLen - 1] &= 127; // 0b0111_1111
+    // u[fieldLen-1] crashes QuickJS (TypeError: out-of-bound numeric index)
+    if (fieldLen === montgomeryBytes) u[fieldLen - 1] &= 127; // 0b0111_1111
     return bytesToNumberLE(u);
   }
   function decodeScalar(n: Hex): bigint {