diff --git a/core/src/main/java/org/lflang/federated/extensions/CExtensionUtils.java b/core/src/main/java/org/lflang/federated/extensions/CExtensionUtils.java
index 541ff67f74..ad982431a2 100644
--- a/core/src/main/java/org/lflang/federated/extensions/CExtensionUtils.java
+++ b/core/src/main/java/org/lflang/federated/extensions/CExtensionUtils.java
@@ -313,6 +313,7 @@ public static void generateCMakeInclude(
         "add_compile_definitions(LF_PACKAGE_DIRECTORY=\"" + fileConfig.srcPkgPath + "\")");
     cmakeIncludeCode.pr(
         "add_compile_definitions(LF_SOURCE_GEN_DIRECTORY=\"" + fileConfig.getSrcGenPath() + "\")");
+    cmakeIncludeCode.pr("add_compile_definitions(LF_FILE_SEPARATOR=\"" + File.separator + "\")");
     try (var srcWriter = Files.newBufferedWriter(cmakeIncludePath)) {
       srcWriter.write(cmakeIncludeCode.getCode());
     }