diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 2d0282389..cc2799910 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -147,7 +147,7 @@ endif()
 
 # If glslag was cloned, then SPIRV/GlslangToSpv.h will be used instead of glslang/SPIRV/GlslangToSpv.h
 if(KOMPUTE_OPT_REPO_SUBMODULE_BUILD)
-        set(USE_EXTERNAL_SHADER 1)
+        add_compile_definitions(USE_EXTERNAL_SHADER=1)
 endif()
 
 if(NOT KOMPUTE_OPT_DISABLE_SHADER_UTILS)