diff --git a/check-diff.sh b/check-diff.sh
index 07d6b57..06697d0 100755
--- a/check-diff.sh
+++ b/check-diff.sh
@@ -36,6 +36,7 @@ function set_environment_variables {
 	PROJECT_SLUG=${PROJECT_SLUG:-$( basename "$PROJECT_DIR" | sed 's/^wp-//' )}
 	PATH_INCLUDES=${PATH_INCLUDES:-./}
 	PATH_EXCLUDES_PATTERN=${PATH_EXCLUDES_PATTERN:-'^(.*/)?(vendor|bower_components|node_modules)/.*'}
+	DEFAULT_BASE_BRANCH=${DEFAULT_BASE_BRANCH:-master}
 
 	if [ -z "$PROJECT_TYPE" ]; then
 		if [ -e style.css ]; then
@@ -57,17 +58,18 @@ function set_environment_variables {
 
 	if [ "$TRAVIS" == true ]; then
 		if [[ "$TRAVIS_PULL_REQUEST" != 'false' ]]; then
-
-			# Make sure the remote branch is fetched.
-			if [[ -z "$DIFF_BASE" ]] && ! git rev-parse --verify --quiet "$TRAVIS_BRANCH"; then
-				git fetch origin "$TRAVIS_BRANCH"
-				git branch "$TRAVIS_BRANCH" FETCH_HEAD
-			fi
-
-			DIFF_BASE=${DIFF_BASE:-$TRAVIS_BRANCH}
+			DIFF_BASE_BRANCH=$TRAVIS_BRANCH
 		else
-			DIFF_BASE=${DIFF_BASE:-$TRAVIS_COMMIT^}
+			DIFF_BASE_BRANCH=$DEFAULT_BASE_BRANCH
 		fi
+
+		# Make sure the remote branch is fetched.
+		if [[ -z "$DIFF_BASE" ]] && ! git rev-parse --verify --quiet "$DIFF_BASE_BRANCH" > /dev/null; then
+			git fetch origin "$DIFF_BASE_BRANCH"
+			git branch "$DIFF_BASE_BRANCH" FETCH_HEAD
+		fi
+
+		DIFF_BASE=${DIFF_BASE:-$DIFF_BASE_BRANCH}
 		DIFF_HEAD=${DIFF_HEAD:-$TRAVIS_COMMIT}
 	else
 		DIFF_BASE=${DIFF_BASE:-HEAD}
diff --git a/readme.md b/readme.md
index 37151b3..fa3e4a1 100644
--- a/readme.md
+++ b/readme.md
@@ -128,6 +128,7 @@ You may customize the behavior of the `.travis.yml` and `pre-commit` hook by
 specifying a `.dev-lib` (formerly `.ci-env.sh`) Bash script in the root of the repo, for example:
 
 ```bash
+DEFAULT_BASE_BRANCH=develop
 PHPCS_GITHUB_SRC=xwp/PHP_CodeSniffer
 PHPCS_GIT_TREE=phpcs-patch
 PHPCS_IGNORE='tests/*,includes/vendor/*' # See also PATH_INCLUDES below
@@ -139,7 +140,7 @@ PATH_INCLUDES="docroot/wp-content/plugins/acme-* docroot/wp-content/themes/acme-
 CHECK_SCOPE=patches
 ```
 
-The `PATH_INCLUDES` is especially useful when the dev-lib is used in the context of an entire site, so you can target just the themes and plugins that you're responsible for. For *excludes*, you can specify a `PHPCS_IGNORE` var and override the `.jshintignore` (it would be better to have a `PATH_EXCLUDES` as well).
+Set `DEFAULT_BASE_BRANCH` to be whatever your default branch is in GitHub; this is use when doing diff-checks on changes in a branch build on Travis CI. The `PATH_INCLUDES` is especially useful when the dev-lib is used in the context of an entire site, so you can target just the themes and plugins that you're responsible for. For *excludes*, you can specify a `PHPCS_IGNORE` var and override the `.jshintignore`; there is a `PATH_EXCLUDES_PATTERN` as well.
 
 ## Pre-commit tips