diff --git a/bin/pyenv-sh-activate b/bin/pyenv-sh-activate
index d6b0d35..38091ee 100755
--- a/bin/pyenv-sh-activate
+++ b/bin/pyenv-sh-activate
@@ -96,8 +96,8 @@ fi
 venv="${versions}"
 
 if [ -n "${VIRTUAL_ENV}" ]; then
-  # exit as success if some virtualenv is already activated outside from pyenv-virtualenv
-  if [ -z "${PYENV_VIRTUAL_ENV}" ]; then
+  # exit as success if a non-pyenv virtualenv is active
+  if [[ -z $PYENV_VIRTUAL_ENV || $PYENV_VIRTUAL_ENV != "$VIRTUAL_ENV" ]]; then
     if [ -z "${FORCE}" ]; then
       if [ -z "${QUIET}" ]; then
         echo "pyenv-virtualenv: virtualenv \`${VIRTUAL_ENV}' is already activated" 1>&2
diff --git a/test/activate.bats b/test/activate.bats
index 9c1fc67..8c1760a 100644
--- a/test/activate.bats
+++ b/test/activate.bats
@@ -439,6 +439,34 @@ EOS
   unstub pyenv-prefix
 }
 
+@test "do nothing if a 3rd-party virtualenv is active" {
+  export PYENV_VIRTUALENV_INIT=1
+  export VIRTUAL_ENV="${TMP}/venv-3rd-party"
+  unset PYENV_VIRTUAL_ENV
+
+  PYENV_SHELL="bash" run pyenv-sh-activate "venv"
+
+  assert_success
+  assert_output <<EOS
+pyenv-virtualenv: virtualenv \`${TMP}/venv-3rd-party' is already activated
+true
+EOS
+}
+
+@test "do nothing if a 3rd-party virtualenv is active over ours" {
+  export PYENV_VIRTUALENV_INIT=1
+  export VIRTUAL_ENV="${TMP}/venv-3rd-party"
+  export PYENV_VIRTUAL_ENV="${PYENV_ROOT}/versions/venv"
+
+  PYENV_SHELL="bash" run pyenv-sh-activate "venv"
+
+  assert_success
+  assert_output <<EOS
+pyenv-virtualenv: virtualenv \`${TMP}/venv-3rd-party' is already activated
+true
+EOS
+}
+
 @test "should fail if activate is invoked as a command" {
   run pyenv-activate
 
diff --git a/test/stubs/stub b/test/stubs/stub
index bd5c5d6..a3b0f33 100755
--- a/test/stubs/stub
+++ b/test/stubs/stub
@@ -11,11 +11,10 @@ _STUB_RUN="${PROGRAM}_STUB_RUN"
 _STUB_INDEX="${PROGRAM}_STUB_INDEX"
 _STUB_RESULT="${PROGRAM}_STUB_RESULT"
 _STUB_END="${PROGRAM}_STUB_END"
-_STUB_DEBUG="${PROGRAM}_STUB_DEBUG"
+_STUB_LOG="${PROGRAM}_STUB_LOG"
 
-if [ -n "${!_STUB_DEBUG}" ]; then
-  echo "$program" "$@" >&${!_STUB_DEBUG}
-fi
+[ -n "${!_STUB_LOG}" ] || eval "${_STUB_LOG}"="${TMPDIR}/${program}-stub-log"
+if test -z "${!_STUB_END}"; then echo "$program" "$@" >>"${!_STUB_LOG}"; fi
 
 [ -e "${!_STUB_PLAN}" ] || exit 1
 [ -n "${!_STUB_RUN}" ] || eval "${_STUB_RUN}"="${TMPDIR}/${program}-stub-run"
@@ -24,7 +23,7 @@ fi
 # Initialize or load the stub run information.
 eval "${_STUB_INDEX}"=1
 eval "${_STUB_RESULT}"=0
-[ ! -e "${!_STUB_RUN}" ] || source "${!_STUB_RUN}"
+if test -e "${!_STUB_RUN}"; then source "${!_STUB_RUN}"; fi
 
 
 # Loop over each line in the plan.
@@ -80,14 +79,26 @@ done < "${!_STUB_PLAN}"
 
 
 if [ -n "${!_STUB_END}" ]; then
-  # Clean up the run file.
-  rm -f "${!_STUB_RUN}"
-
   # If the number of lines in the plan is larger than
   # the requested index, we failed.
   if [ $index -ge "${!_STUB_INDEX}" ]; then
     eval "${_STUB_RESULT}"=1
   fi
+  if [ "${!_STUB_RESULT}" -ne 0 ]; then
+    {
+      echo "index: $index; stub index: ${!_STUB_INDEX}"
+      echo "plan:"
+      cat "${!_STUB_PLAN}" || true
+      echo "run:"
+      cat "${!_STUB_RUN}" || true
+      echo "log:"
+      cat "${!_STUB_LOG}" || true
+    } >&2
+  fi
+
+  # Clean up the run file.
+  rm -f "${!_STUB_RUN}"
+  rm -f "${!_STUB_LOG}"
 
   # Return the result.
   exit "${!_STUB_RESULT}"
diff --git a/test/test_helper.bash b/test/test_helper.bash
index 08bcb79..d63b874 100644
--- a/test/test_helper.bash
+++ b/test/test_helper.bash
@@ -16,6 +16,7 @@ stub() {
 
   export "${prefix}_STUB_PLAN"="${TMP}/${program}-stub-plan"
   export "${prefix}_STUB_RUN"="${TMP}/${program}-stub-run"
+  export "${prefix}_STUB_LOG"="${TMP}/${program}-stub-log"
   export "${prefix}_STUB_END"=
 
   mkdir -p "${TMP}/bin"