diff --git a/lib/src/protofier.ts b/lib/src/protofier.ts
index e5d487cb..e2d7f951 100644
--- a/lib/src/protofier.ts
+++ b/lib/src/protofier.ts
@@ -24,7 +24,7 @@ import {
   CalculationOperation,
   CalculationOperator,
 } from './value/calculations';
-import { SassMixin } from './value/mixin';
+import {SassMixin} from './value/mixin';
 
 /**
  * A class that converts [Value] objects into protobufs.
@@ -329,7 +329,6 @@ export class Protofier {
 
       case 'compilerMixin':
         return new SassMixin(value.value.value.id);
-  
 
       case 'calculation':
         return this.deprotofyCalculation(value.value.value);
diff --git a/lib/src/value/mixin.ts b/lib/src/value/mixin.ts
index 2198f8f6..1d8ecd86 100644
--- a/lib/src/value/mixin.ts
+++ b/lib/src/value/mixin.ts
@@ -9,14 +9,14 @@ import {Value} from './index';
 /** A first-class SassScript mixin. */
 export class SassMixin extends Value {
   /**
-   * If this mixin is defined in the compiler, this is the unique ID that the
-   * compiler uses to determine which mixin it refers to.
+   * This is the unique ID that the compiler uses to determine which mixin it
+   * refers to.
    *
    * This is marked as public so that the protofier can access it, but it's not
    * part of the package's public API and should not be accessed by user code.
    * It may be renamed or removed without warning in the future.
    */
-  readonly id: number | undefined;
+  readonly id: number;
 
   constructor(id: number) {
     super();
@@ -24,9 +24,7 @@ export class SassMixin extends Value {
   }
 
   equals(other: Value): boolean {
-    return this.id === undefined
-      ? other === this
-      : other instanceof SassMixin && other.id === this.id;
+    return other instanceof SassMixin && other.id === this.id;
   }
 
   hashCode(): number {