diff --git a/index.js b/index.js
index 79dd017..8f8254c 100644
--- a/index.js
+++ b/index.js
@@ -42,7 +42,9 @@ function Babel(inputTree, _options) {
   delete options.description;
 
   this.console = options.console || console;
+  this.throwUnlessParallelizable = options.throwUnlessParallelizable;
   delete options.console;
+  delete options.throwUnlessParallelizable;
 
   this.options = options;
   this.extensions = this.options.filterExtensions || ['js'];
@@ -78,7 +80,9 @@ Babel.prototype.baseDir = function() {
 };
 
 Babel.prototype.transform = function(string, options) {
-  return transformString(string, options);
+  return transformString(string, options, {
+    throwUnlessParallelizable: this.throwUnlessParallelizable
+  });
 };
 
 /*
diff --git a/lib/parallel-api.js b/lib/parallel-api.js
index 90a011c..0d84f56 100644
--- a/lib/parallel-api.js
+++ b/lib/parallel-api.js
@@ -96,8 +96,12 @@ function deserializeOptions(options) {
 
   // transform callbacks
   Object.keys(options).forEach(key => {
-    if (implementsParallelAPI(options[key])) {
-      newOptions[key] = buildFromParallelApiInfo(options[key]._parallelBabel);
+    let value = options[key];
+    if (value === undefined) {
+      return;
+    }
+    if (implementsParallelAPI(value)) {
+      newOptions[key] = buildFromParallelApiInfo(value._parallelBabel);
     }
   });
 
@@ -129,17 +133,17 @@ function serializeOptions(options) {
   return serializableOptions;
 }
 
-function transformString(string, options) {
-  const isParallelizable = transformIsParallelizable(options);
+function transformString(string, babelOptions, buildOptions) {
+  const isParallelizable = transformIsParallelizable(babelOptions);
 
-  if (options.throwUnlessParallelizable && !isParallelizable) {
+  if (buildOptions !== null && typeof buildOptions === 'object' && buildOptions.throwUnlessParallelizable && !isParallelizable) {
     throw new Error('BroccoliBabelTranspiler was configured to `throwUnlessParallelizable` and was unable to parallelize an plugin. Please see: https://github.com/babel/broccoli-babel-transpiler#parallel-transpilation for more details');
   }
 
   if (JOBS > 1 && isParallelizable) {
     let pool = getWorkerPool();
     _logger.debug('transformString is parallelizable');
-    return pool.exec('transform', [string, serializeOptions(options)]);
+    return pool.exec('transform', [string, serializeOptions(babelOptions)]);
   } else {
     if (JOBS <= 1) {
       _logger.debug('JOBS <= 1, skipping worker, using main thread');
@@ -148,7 +152,7 @@ function transformString(string, options) {
     }
 
     return new Promise(resolve => {
-      resolve(transpiler.transform(string, deserializeOptions(options)));
+      resolve(transpiler.transform(string, deserializeOptions(babelOptions)));
     });
   }
 }
diff --git a/tests/test.js b/tests/test.js
index 4b70914..c0225ba 100644
--- a/tests/test.js
+++ b/tests/test.js
@@ -1334,27 +1334,24 @@ describe('workerpool', function() {
 
   describe('throwUnlessParallelizable', function() {
     it('should throw if throwUnlessParallelizable: true, and one or more plugins could not be parallelized', function() {
-      options.throwUnlessParallelizable = true;
       options.plugins = [function() { }];
-      expect(() => require(PATH).transformString(stringToTransform, options)).to.throw(/BroccoliBabelTranspiler was configured to `throwUnlessParallelizable` and was unable to parallelize/);
+      expect(() => require(PATH).transformString(stringToTransform, options, { throwUnlessParallelizable: true })).to.throw(/BroccoliBabelTranspiler was configured to `throwUnlessParallelizable` and was unable to parallelize/);
     });
 
     it('should NOT throw if throwUnlessParallelizable: true, and all plugins can be parallelized', function() {
-      options.throwUnlessParallelizable = true;
       options.plugins = [ { foo: 1 }];
-      expect(() => require(PATH).transformString(stringToTransform, options)).to.throw(/BroccoliBabelTranspiler was configured to `throwUnlessParallelizable` and was unable to parallelize/);
+      expect(() => require(PATH).transformString(stringToTransform, options, { throwUnlessParallelizable: true })).to.throw(/BroccoliBabelTranspiler was configured to `throwUnlessParallelizable` and was unable to parallelize/);
     });
 
     it('should NOT throw if throwUnlessParallelizable: false, and one or more plugins could not be parallelized', function() {
-      options.throwUnlessParallelizable = false
       options.plugins = [function() { }]
-      expect(() => require(PATH).transformString(stringToTransform, options)).to.not.throw();
+      expect(() => require(PATH).transformString(stringToTransform, options, { throwUnlessParallelizable:  false })).to.not.throw();
     });
 
     it('should NOT throw if throwUnlessParallelizable is unset, and one or more plugins could not be parallelized', function() {
-      delete options.throwUnlessParallelizable;
-      options.plugins = [function() { }]
-      expect(() => require(PATH).transformString(stringToTransform, options)).to.not.throw();
+      expect(() => require(PATH).transformString(stringToTransform, options, { throwUnlessParallelizable: undefined })).to.not.throw();
+      expect(() => require(PATH).transformString(stringToTransform, options, { throwUnlessParallelizable: null })).to.not.throw();
+      expect(() => require(PATH).transformString(stringToTransform, options, { })).to.not.throw();
     });
   });
 });