diff --git a/src/core/evaluator.js b/src/core/evaluator.js
index 27c7256a3..2c3c80964 100644
--- a/src/core/evaluator.js
+++ b/src/core/evaluator.js
@@ -78,8 +78,24 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
       if (this.builtInCMapCache.has(name)) {
         return this.builtInCMapCache.get(name);
       }
-      const data = await this.handler.sendWithPromise('FetchBuiltInCMap',
-                                                      { name, });
+      const readableStream = this.handler.sendWithStream('FetchBuiltInCMap', {
+        name,
+      });
+      const reader = readableStream.getReader();
+
+      const data = await new Promise(function(resolve, reject) {
+        function pump() {
+          reader.read().then(function({ value, done, }) {
+            if (done) {
+              return;
+            }
+            resolve(value);
+            pump();
+          }, reject);
+        }
+        pump();
+      });
+
       if (data.compressionType !== CMapCompressionType.NONE) {
         // Given the size of uncompressed CMaps, only cache compressed ones.
         this.builtInCMapCache.set(name, data);
diff --git a/src/display/api.js b/src/display/api.js
index a964d52ff..1be287ac6 100644
--- a/src/display/api.js
+++ b/src/display/api.js
@@ -2230,11 +2230,26 @@ class WorkerTransport {
       });
     });
 
-    messageHandler.on('FetchBuiltInCMap', (data) => {
+    messageHandler.on('FetchBuiltInCMap', (data, sink) => {
       if (this.destroyed) {
-        return Promise.reject(new Error('Worker was destroyed'));
+        sink.error(new Error('Worker was destroyed'));
+        return;
       }
-      return this.CMapReaderFactory.fetch(data);
+      let fetched = false;
+
+      sink.onPull = () => {
+        if (fetched) {
+          sink.close();
+          return;
+        }
+        fetched = true;
+
+        this.CMapReaderFactory.fetch(data).then(function(builtInCMap) {
+          sink.enqueue(builtInCMap, 1, [builtInCMap.cMapData.buffer]);
+        }).catch(function(reason) {
+          sink.error(reason);
+        });
+      };
     });
   }