diff --git a/linux-tkg-config/prepare b/linux-tkg-config/prepare
index 893c37a..4721f16 100644
--- a/linux-tkg-config/prepare
+++ b/linux-tkg-config/prepare
@@ -659,7 +659,7 @@ _tkg_srcprep() {
 
   # PREEMPT_RT patch
   if [ "${_preempt_rt}" = "1" ]; then
-    if [ ${_rt_subver_map[$_basekernel]+_} = "_" ]; then
+    if [ "${_rt_subver_map[$_basekernel]+_}" = "_" ]; then
       preempt_rt_ksubver="${_rt_subver_map[$_basekernel]}"
       # Check if subversion is supported, skip check if forced
       if [ "${_preempt_rt_force}" = "1" ] || [ "${preempt_rt_ksubver}" = "${_sub}" ]; then