diff --git a/.travis.yml b/.travis.yml
index 8fa89666..777c536e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,4 +11,6 @@ cache:
   directories:
     - "node_modules"
 script:
+  - ./checkgit.sh "Source files were modified before build; is yarn.lock out of sync with package.json?" || travis_terminate $?
   - yarn grunt
+  - ./checkgit.sh "Source files were modified by the build" || travis_terminate $?
diff --git a/checkgit.sh b/checkgit.sh
new file mode 100755
index 00000000..ee29eb4b
--- /dev/null
+++ b/checkgit.sh
@@ -0,0 +1,6 @@
+GIT_STATUS=$(git status --porcelain | wc -l )
+if [[ GIT_STATUS -ne 0 ]]; then
+  echo "${1:-Source files were modified}"
+  git status
+  exit $GIT_STATUS
+fi;