diff --git a/packages/angular_devkit/build_angular/src/karma/schema.json b/packages/angular_devkit/build_angular/src/karma/schema.json
index 4316a48ca2..07c55a4134 100644
--- a/packages/angular_devkit/build_angular/src/karma/schema.json
+++ b/packages/angular_devkit/build_angular/src/karma/schema.json
@@ -64,7 +64,8 @@
     },
     "sourceMap": {
       "type": "boolean",
-      "description": "Output sourcemaps."
+      "description": "Output sourcemaps.",
+      "default": true
     },
     "vendorSourceMap": {
       "type": "boolean",