diff --git a/t/perf/perf-lib.sh b/t/perf/perf-lib.sh
index 9cdccba..b58a43e 100644
--- a/t/perf/perf-lib.sh
+++ b/t/perf/perf-lib.sh
@@ -27,6 +27,17 @@
 
 . ../test-lib.sh
 
+if test -n "$GIT_TEST_INSTALLED" -a -z "$PERF_SET_GIT_TEST_INSTALLED"
+then
+	error "Do not use GIT_TEST_INSTALLED with the perf tests.
+
+Instead use:
+
+    ./run <path-to-git> -- <tests>
+
+See t/perf/README for details."
+fi
+
 # Variables from test-lib that are normally internal to the tests; we
 # need to export them for test_perf subshells
 export TEST_DIRECTORY TRASH_DIRECTORY GIT_BUILD_DIR GIT_TEST_CMP
diff --git a/t/perf/run b/t/perf/run
index cd3882b..c7b8610 100755
--- a/t/perf/run
+++ b/t/perf/run
@@ -84,6 +84,8 @@
 		GIT_TEST_INSTALLED=$mydir_abs
 	fi
 	export GIT_TEST_INSTALLED
+	PERF_SET_GIT_TEST_INSTALLED=true
+	export PERF_SET_GIT_TEST_INSTALLED
 }
 
 run_dirs_helper () {