diff --git a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
index 65b93f2731ea..04bddef732f4 100644
--- a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
+++ b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
@@ -54,6 +54,11 @@ function isKnownCoreFile(filePath: string) {
 }
 
 function isKnownSideEffectFree(filePath: string) {
+  // rxjs add imports contain intentional side effects
+  if (/[\\/]node_modules[\\/]rxjs[\\/]add[\\/]/.test(filePath)) {
+    return false;
+  }
+
   return ngFactories.some((s) => filePath.endsWith(s)) ||
     knownSideEffectFreeAngularModules.some((re) => re.test(filePath));
 }