diff --git a/ament_cmake_test/cmake/ament_add_test.cmake b/ament_cmake_test/cmake/ament_add_test.cmake
index a7dcea35..b4b6cdb1 100644
--- a/ament_cmake_test/cmake/ament_add_test.cmake
+++ b/ament_cmake_test/cmake/ament_add_test.cmake
@@ -72,12 +72,11 @@ function(ament_add_test testname)
   if(NOT ARG_RUNNER)
     set(ARG_RUNNER "${ament_cmake_test_DIR}/run_test.py")
   endif()
-  if(NOT ARG_TIMEOUT)
+  if(NOT DEFINED ARG_TIMEOUT OR "${ARG_TIMEOUT}" STREQUAL "")
     set(ARG_TIMEOUT 60)
   endif()
-  if(NOT ARG_TIMEOUT GREATER 0)
-    message(FATAL_ERROR "ament_add_test() the TIMEOUT argument must be a "
-      "valid number and greater than zero")
+  if(NOT ARG_TIMEOUT GREATER_EQUAL 0)
+    message(FATAL_ERROR "ament_add_test() the TIMEOUT argument must be a positive number OR zero (for unlimited time)")
   endif()
   if(NOT ARG_WORKING_DIRECTORY)
     set(ARG_WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}")