Fixing cmake issue with empty cmake_build_type

test/CMakeLists.txt

diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 5a63b17..7e4f485 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -45,7 +45,9 @@
 add_test(cxx03 cxx03_test --benchmark_min_time=0.01)
 
 # Add the coverage command(s)
-string(TOLOWER ${CMAKE_BUILD_TYPE} CMAKE_BUILD_TYPE_LOWER)
+if(CMAKE_BUILD_TYPE)
+  string(TOLOWER ${CMAKE_BUILD_TYPE} CMAKE_BUILD_TYPE_LOWER)
+endif()
 if (${CMAKE_BUILD_TYPE_LOWER} MATCHES "coverage")
   find_program(GCOV gcov)
   find_program(LCOV lcov)