diff --git a/ci/lib.sh b/ci/lib.sh
index 16f4ecb..288a5b3 100755
--- a/ci/lib.sh
+++ b/ci/lib.sh
@@ -74,6 +74,9 @@
 	}
 }
 
+# Clear MAKEFLAGS that may come from the outside world.
+export MAKEFLAGS=
+
 # Set 'exit on error' for all CI scripts to let the caller know that
 # something went wrong.
 # Set tracing executed commands, primarily setting environment variables
@@ -101,7 +104,7 @@
 	BREW_INSTALL_PACKAGES="git-lfs gettext"
 	export GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
 	export GIT_TEST_OPTS="--verbose-log -x --immediate"
-	export MAKEFLAGS="--jobs=2"
+	MAKEFLAGS="$MAKEFLAGS --jobs=2"
 elif test -n "$SYSTEM_COLLECTIONURI" || test -n "$SYSTEM_TASKDEFINITIONSURI"
 then
 	CI_TYPE=azure-pipelines
@@ -126,7 +129,7 @@
 	BREW_INSTALL_PACKAGES=gcc@8
 	export GIT_PROVE_OPTS="--timer --jobs 10 --state=failed,slow,save"
 	export GIT_TEST_OPTS="--verbose-log -x --write-junit-xml"
-	export MAKEFLAGS="--jobs=10"
+	MAKEFLAGS="$MAKEFLAGS --jobs=10"
 	test windows_nt != "$CI_OS_NAME" ||
 	GIT_TEST_OPTS="--no-chain-lint --no-bin-wrappers $GIT_TEST_OPTS"
 else
@@ -185,4 +188,4 @@
 	;;
 esac
 
-export MAKEFLAGS="CC=${CC:-cc}"
+MAKEFLAGS="$MAKEFLAGS CC=${CC:-cc}"