diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 19e67710..27f7f1b0 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -67,15 +67,15 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
     )
 endif()
 
-if(UNITS_DOWNCAST_MODE STREQUAL "AUTO")
-    message(STATUS "Configuring UNITS_DOWNCAST_MODE=AUTOMATIC")
-    target_compile_definitions(mp-units INTERFACE UNITS_DOWNCAST_MODE=2)
-elseif(UNITS_DOWNCAST_MODE)
-    message(STATUS "Configuring UNITS_DOWNCAST_MODE=ON")
-    target_compile_definitions(mp-units INTERFACE UNITS_DOWNCAST_MODE=1)
-else()
-    message(STATUS "Configuring UNITS_DOWNCAST_MODE=OFF")
-    target_compile_definitions(mp-units INTERFACE UNITS_DOWNCAST_MODE=0)
+if(DEFINED UNITS_DOWNCAST_MODE)
+    set(downcast_mode_options OFF ON AUTO)
+    list(FIND downcast_mode_options "${UNITS_DOWNCAST_MODE}" downcast_mode)
+    if(downcast_mode EQUAL -1)
+        message(FATAL_ERROR "'${UNITS_DOWNCAST_MODE}' is an invalid UNITS_DOWNCAST_MODE value")
+    else()
+        message(STATUS "Configuring UNITS_DOWNCAST_MODE=${UNITS_DOWNCAST_MODE}")
+        target_compile_definitions(mp-units INTERFACE UNITS_DOWNCAST_MODE=${downcast_mode})
+    endif()
 endif()
 
 add_library(mp::units ALIAS mp-units)