diff --git a/libexec/rbenv-prefix b/libexec/rbenv-prefix
index 5376cf5..e65be83 100755
--- a/libexec/rbenv-prefix
+++ b/libexec/rbenv-prefix
@@ -24,7 +24,8 @@ fi
 if [ "$RBENV_VERSION" = "system" ]; then
   if RUBY_PATH="$(rbenv-which ruby 2>/dev/null)"; then
     RUBY_PATH="${RUBY_PATH%/*}"
-    echo "${RUBY_PATH%/bin}"
+    RBENV_PREFIX_PATH="${RUBY_PATH%/bin}"
+    echo "${RBENV_PREFIX_PATH:-/}"
     exit
   else
     echo "rbenv: system version not found in PATH" >&2
diff --git a/test/prefix.bats b/test/prefix.bats
index 3fb9408..bdee6f0 100644
--- a/test/prefix.bats
+++ b/test/prefix.bats
@@ -24,6 +24,18 @@ load test_helper
   assert_success "$RBENV_TEST_DIR"
 }
 
+@test "prefix for system in /" {
+  mkdir -p "${BATS_TEST_DIRNAME}/libexec"
+  cat >"${BATS_TEST_DIRNAME}/libexec/rbenv-which" <<OUT
+#!/bin/sh
+echo /bin/ruby
+OUT
+  chmod +x "${BATS_TEST_DIRNAME}/libexec/rbenv-which"
+  RBENV_VERSION="system" run rbenv-prefix
+  assert_success "/"
+  rm -f "${BATS_TEST_DIRNAME}/libexec/rbenv-which"
+}
+
 @test "prefix for invalid system" {
   PATH="$(path_without ruby)" run rbenv-prefix system
   assert_failure "rbenv: system version not found in PATH"