diff --git a/bin/pyenv-virtualenv-init b/bin/pyenv-virtualenv-init
index 1a60c53..a24a785 100755
--- a/bin/pyenv-virtualenv-init
+++ b/bin/pyenv-virtualenv-init
@@ -129,7 +129,7 @@ esac
 if [[ "$shell" != "fish" ]]; then
   cat <<EOS
   local ret=\$?
-  if [ -n "\$VIRTUAL_ENV" ]; then
+  if [ -n "\${VIRTUAL_ENV-}" ]; then
     eval "\$(pyenv sh-activate --quiet || pyenv sh-deactivate --quiet || true)" || true
   else
     eval "\$(pyenv sh-activate --quiet || true)" || true
@@ -141,8 +141,8 @@ EOS
   case "$shell" in
   bash )
     cat <<EOS
-if ! [[ "\$PROMPT_COMMAND" =~ _pyenv_virtualenv_hook ]]; then
-  PROMPT_COMMAND="_pyenv_virtualenv_hook;\$PROMPT_COMMAND";
+if ! [[ "\${PROMPT_COMMAND-}" =~ _pyenv_virtualenv_hook ]]; then
+  PROMPT_COMMAND="_pyenv_virtualenv_hook;\${PROMPT_COMMAND-}"
 fi
 EOS
     ;;
diff --git a/test/init.bats b/test/init.bats
index 74b0e2b..ba90b5d 100644
--- a/test/init.bats
+++ b/test/init.bats
@@ -6,7 +6,7 @@ load test_helper
   unset PYENV_SHELL
   SHELL=/bin/false run pyenv-virtualenv-init -
   assert_success
-  assert_output_contains '  PROMPT_COMMAND="_pyenv_virtualenv_hook;$PROMPT_COMMAND";'
+  assert_output_contains '  PROMPT_COMMAND="_pyenv_virtualenv_hook;${PROMPT_COMMAND-}"'
 }
 
 @test "detect parent shell from script (sh)" {
@@ -15,7 +15,7 @@ load test_helper
   chmod +x ${TMP}/script.sh
   run ${TMP}/script.sh
   assert_success
-  assert_output_contains_not '  PROMPT_COMMAND="_pyenv_virtualenv_hook;$PROMPT_COMMAND";'
+  assert_output_contains_not '  PROMPT_COMMAND="_pyenv_virtualenv_hook;${PROMPT_COMMAND-}"'
   rm -f "${TMP}/script.sh"
 }
 
@@ -25,7 +25,7 @@ load test_helper
   chmod +x ${TMP}/script.sh
   run ${TMP}/script.sh
   assert_success
-  assert_output_contains '  PROMPT_COMMAND="_pyenv_virtualenv_hook;$PROMPT_COMMAND";'
+  assert_output_contains '  PROMPT_COMMAND="_pyenv_virtualenv_hook;${PROMPT_COMMAND-}"'
   rm -f "${TMP}/script.sh"
 }
 
@@ -54,15 +54,15 @@ export PATH="${TMP}/pyenv/plugins/pyenv-virtualenv/shims:\${PATH}";
 export PYENV_VIRTUALENV_INIT=1;
 _pyenv_virtualenv_hook() {
   local ret=\$?
-  if [ -n "\$VIRTUAL_ENV" ]; then
+  if [ -n "\${VIRTUAL_ENV-}" ]; then
     eval "\$(pyenv sh-activate --quiet || pyenv sh-deactivate --quiet || true)" || true
   else
     eval "\$(pyenv sh-activate --quiet || true)" || true
   fi
   return \$ret
 };
-if ! [[ "\$PROMPT_COMMAND" =~ _pyenv_virtualenv_hook ]]; then
-  PROMPT_COMMAND="_pyenv_virtualenv_hook;\$PROMPT_COMMAND";
+if ! [[ "\${PROMPT_COMMAND-}" =~ _pyenv_virtualenv_hook ]]; then
+  PROMPT_COMMAND="_pyenv_virtualenv_hook;\${PROMPT_COMMAND-}"
 fi
 EOS
 }
@@ -95,7 +95,7 @@ export PATH="${TMP}/pyenv/plugins/pyenv-virtualenv/shims:\${PATH}";
 export PYENV_VIRTUALENV_INIT=1;
 _pyenv_virtualenv_hook() {
   local ret=\$?
-  if [ -n "\$VIRTUAL_ENV" ]; then
+  if [ -n "\${VIRTUAL_ENV-}" ]; then
     eval "\$(pyenv sh-activate --quiet || pyenv sh-deactivate --quiet || true)" || true
   else
     eval "\$(pyenv sh-activate --quiet || true)" || true