diff --git a/packages/coffeescript/plugin/compile-coffeescript.js b/packages/coffeescript/plugin/compile-coffeescript.js
index a64652ffc3..b23661865a 100644
--- a/packages/coffeescript/plugin/compile-coffeescript.js
+++ b/packages/coffeescript/plugin/compile-coffeescript.js
@@ -83,12 +83,19 @@ export class CoffeeCompiler extends CachingCompiler {
           doubleRoastedCoffee.data != null) {
         output.js = doubleRoastedCoffee.data;
 
-        // Combine the original CoffeeScript source map with the one
-        // produced by this.babelCompiler.processOneFileForTarget.
-        const smg = new SourceMapGenerator(
-          new SourceMapConsumer(doubleRoastedCoffee.sourceMap));
-        smg.applySourceMap(new SourceMapConsumer(sourceMap));
-        sourceMap = smg.toJSON();
+        if (doubleRoastedCoffee.sourceMap) {
+          // Combine the original CoffeeScript source map with the one
+          // produced by this.babelCompiler.processOneFileForTarget.
+          const smg = new SourceMapGenerator(
+            new SourceMapConsumer(doubleRoastedCoffee.sourceMap));
+          smg.applySourceMap(new SourceMapConsumer(sourceMap));
+          sourceMap = smg.toJSON();
+        } else {
+          // If the .coffee file is contained by a node_modules directory,
+          // then BabelCompiler will not transpile it, and there will be
+          // no sourceMap, but that's fine because the original
+          // CoffeeScript sourceMap will still be valid.
+        }
       }
     }