diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6b6fb85841c8..ea09ff65fc42 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -23,8 +23,8 @@ option(BUILD_SHARED_LIBS "Build shared libraries" ON)
 
 # Warn if the user did not set a build type and is using a single-configuration generator.
 get_property(IS_MULTI_CONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
-if (NOT IS_MULTI_CONFIG AND NOT DEFINED CMAKE_BUILD_TYPE)
-    message(WARNING "Single-configuration generators require CMAKE_BUILD_TYPE to be set.")
+if (NOT IS_MULTI_CONFIG AND NOT CMAKE_BUILD_TYPE)
+    message(SEND_ERROR "Single-configuration generators require CMAKE_BUILD_TYPE to be set.")
 endif ()
 
 # Windows has file name length restrictions and lacks an RPATH mechanism.