diff --git a/asynciterator.ts b/asynciterator.ts
index 102936d..45e3531 100644
--- a/asynciterator.ts
+++ b/asynciterator.ts
@@ -791,6 +791,9 @@ export function identity<S>(item: S): typeof item {
   return item;
 }
 
+/** Key indicating the current consumer of a source. */
+export const DESTINATION = Symbol('destination');
+
 
 /**
  An iterator that synchronously transforms every item from its source
@@ -821,7 +824,7 @@ export class MappingIterator<S, D = S> extends AsyncIterator<D> {
     }
     // Otherwise, wire up the source for reading
     else {
-      this._source._destination = this;
+      this._source[DESTINATION] = this;
       this._source.on('end', destinationClose);
       this._source.on('error', destinationEmitError);
       this._source.on('readable', destinationSetReadable);
@@ -854,7 +857,7 @@ export class MappingIterator<S, D = S> extends AsyncIterator<D> {
     this._source.removeListener('end', destinationClose);
     this._source.removeListener('error', destinationEmitError);
     this._source.removeListener('readable', destinationSetReadable);
-    delete this._source._destination;
+    delete this._source[DESTINATION];
     if (this._destroySource)
       this._source.destroy();
     super._end(destroy);
@@ -865,7 +868,7 @@ export class MappingIterator<S, D = S> extends AsyncIterator<D> {
 function ensureSourceAvailable<S>(source?: AsyncIterator<S>, allowDestination = false) {
   if (!source || !isFunction(source.read) || !isFunction(source.on))
     throw new TypeError(`Invalid source: ${source}`);
-  if (!allowDestination && (source as any)._destination)
+  if (!allowDestination && (source as any)[DESTINATION])
     throw new Error('The source already has a destination');
   return source as InternalSource<S>;
 }
@@ -1209,7 +1212,7 @@ export class TransformIterator<S, D = S> extends BufferedIterator<D> {
 
     // Validate and set source
     const source = this._source = this._validateSource(value);
-    source._destination = this;
+    source[DESTINATION] = this;
 
     // Close this iterator if the source has already ended
     if (source.done) {
@@ -1324,7 +1327,7 @@ export class TransformIterator<S, D = S> extends BufferedIterator<D> {
       source.removeListener('end', destinationCloseWhenDone);
       source.removeListener('error', destinationEmitError);
       source.removeListener('readable', destinationFillBuffer);
-      delete source._destination;
+      delete source[DESTINATION];
       if (this._destroySource)
         source.destroy();
     }
@@ -1333,20 +1336,20 @@ export class TransformIterator<S, D = S> extends BufferedIterator<D> {
 }
 
 function destinationSetReadable<S>(this: InternalSource<S>) {
-  this._destination!.readable = true;
+  this[DESTINATION]!.readable = true;
 }
 function destinationEmitError<S>(this: InternalSource<S>, error: Error) {
-  this._destination!.emit('error', error);
+  this[DESTINATION]!.emit('error', error);
 }
 function destinationClose<S>(this: InternalSource<S>) {
-  this._destination!.close();
+  this[DESTINATION]!.close();
 }
 function destinationCloseWhenDone<S>(this: InternalSource<S>) {
-  (this._destination as any)._closeWhenDone();
+  (this[DESTINATION] as any)._closeWhenDone();
 }
 function destinationFillBuffer<S>(this: InternalSource<S>) {
-  if ((this._destination as any)._sourceStarted !== false)
-    (this._destination as any)._fillBuffer();
+  if ((this[DESTINATION] as any)._sourceStarted !== false)
+    (this[DESTINATION] as any)._fillBuffer();
 }
 
 
@@ -1564,7 +1567,7 @@ export class MultiTransformIterator<S, D = S> extends TransformIterator<S, D> {
       // Create the transformer and listen to its events
       const transformer = (this._createTransformer(item) ||
         new EmptyIterator()) as InternalSource<D>;
-      transformer._destination = this;
+      transformer[DESTINATION] = this;
       transformer.on('end', destinationFillBuffer);
       transformer.on('readable', destinationFillBuffer);
       transformer.on('error', destinationEmitError);
@@ -1689,7 +1692,7 @@ export class UnionIterator<T> extends BufferedIterator<T> {
       source = wrap<T>(source) as any as InternalSource<T>;
     if (!source.done) {
       this._sources.push(source);
-      source._destination = this;
+      source[DESTINATION] = this;
       source.on('error', destinationEmitError);
       source.on('readable', destinationFillBuffer);
       source.on('end', destinationRemoveEmptySources);
@@ -1753,7 +1756,7 @@ export class UnionIterator<T> extends BufferedIterator<T> {
 }
 
 function destinationRemoveEmptySources<T>(this: InternalSource<T>) {
-  (this._destination as any)._removeEmptySources();
+  (this[DESTINATION] as any)._removeEmptySources();
 }
 
 
@@ -1795,8 +1798,8 @@ export class ClonedIterator<T> extends TransformIterator<T> {
     // Validate and set the source
     const source = this._source = this._validateSource(value);
     // Create a history reader for the source if none already existed
-    const history = (source && (source as any)._destination) ||
-      (source._destination = new HistoryReader<T>(source) as any);
+    const history = (source && (source as any)[DESTINATION]) ||
+      (source[DESTINATION] = new HistoryReader<T>(source) as any);
 
     // Close this clone if history is empty and the source has ended
     if (history.endsAt(0)) {
@@ -1827,7 +1830,7 @@ export class ClonedIterator<T> extends TransformIterator<T> {
     @param {boolean} allowDestination Whether the source can already have a destination
   */
   protected _validateSource(source?: AsyncIterator<T>, allowDestination = false) {
-    const history = (source && (source as any)._destination);
+    const history = (source && (source as any)[DESTINATION]);
     return super._validateSource(source, !history || history instanceof HistoryReader);
   }
 
@@ -1881,7 +1884,7 @@ export class ClonedIterator<T> extends TransformIterator<T> {
     let item = null;
     if (!this.done && source) {
       // Try to read an item at the current point in history
-      const history = source._destination as any as HistoryReader<T>;
+      const history = source[DESTINATION] as any as HistoryReader<T>;
       if ((item = history.readAt(this._readPosition)) !== null)
         this._readPosition++;
       else
@@ -1897,7 +1900,7 @@ export class ClonedIterator<T> extends TransformIterator<T> {
   protected _end(destroy: boolean) {
     // Unregister from a possible history reader
     const source = this.source as InternalSource<T>;
-    const history = source?._destination as any as HistoryReader<T>;
+    const history = source?.[DESTINATION] as any as HistoryReader<T>;
     if (history)
       history.unregister(this);
 
@@ -2043,7 +2046,7 @@ export class WrappingIterator<T> extends AsyncIterator<T> {
     }
 
     // Set up event handling
-    source._destination = this;
+    source[DESTINATION] = this;
     source.on('end', destinationClose);
     source.on('error', destinationEmitError);
     source.on('readable', destinationSetReadable);
@@ -2070,7 +2073,7 @@ export class WrappingIterator<T> extends AsyncIterator<T> {
       this._source.removeListener('end', destinationClose);
       this._source.removeListener('error', destinationEmitError);
       this._source.removeListener('readable', destinationSetReadable);
-      delete this._source._destination;
+      delete this._source[DESTINATION];
       this._source = null;
     }
   }
@@ -2224,7 +2227,7 @@ type SourceExpression<T> =
   (() => MaybePromise<AsyncIterator<T>>);
 
 type InternalSource<T> =
-  AsyncIterator<T> & { _destination?: AsyncIterator<any> };
+  AsyncIterator<T> & { [DESTINATION]?: AsyncIterator<any> };
 
 // Returns a function that calls `fn` with `self` as `this` pointer. */
 function bind<T extends Function>(fn: T, self?: object): T {
diff --git a/test/ClonedIterator-test.js b/test/ClonedIterator-test.js
index f543728..98acefa 100644
--- a/test/ClonedIterator-test.js
+++ b/test/ClonedIterator-test.js
@@ -5,6 +5,7 @@ import {
   BufferedIterator,
   EmptyIterator,
   ArrayIterator,
+  DESTINATION,
 } from '../dist/asynciterator.js';
 
 import { EventEmitter } from 'events';
@@ -120,7 +121,7 @@ describe('ClonedIterator', () => {
   describe('Cloning an iterator that already has a destination', () => {
     it('should throw an exception', () => {
       const source = new AsyncIterator(), destination = new TransformIterator(source);
-      source.should.have.property('_destination', destination);
+      source.should.have.property(DESTINATION, destination);
       (() => source.clone()).should.throw('The source already has a destination');
     });
   });
diff --git a/test/TransformIterator-test.js b/test/TransformIterator-test.js
index 5ad99a0..595775a 100644
--- a/test/TransformIterator-test.js
+++ b/test/TransformIterator-test.js
@@ -6,6 +6,7 @@ import {
   TransformIterator,
   wrap,
   scheduleTask,
+  DESTINATION,
 } from '../dist/asynciterator.js';
 
 import { EventEmitter } from 'events';
@@ -359,7 +360,7 @@ describe('TransformIterator', () => {
       });
 
       it('should remove itself as destination from the source', () => {
-        source.should.not.have.key('_destination');
+        source.should.not.have.key(DESTINATION);
       });
     });
   });
@@ -466,7 +467,7 @@ describe('TransformIterator', () => {
       });
 
       it('should remove itself as destination from the source', () => {
-        source.should.not.have.key('_destination');
+        source.should.not.have.key(DESTINATION);
       });
     });
   });
@@ -575,7 +576,7 @@ describe('TransformIterator', () => {
       });
 
       it('should remove itself as destination from the source', () => {
-        source.should.not.have.key('_destination');
+        source.should.not.have.key(DESTINATION);
       });
     });
   });