diff --git a/lm-c99/tests/test_ig_c99/Makefile b/lm-c99/tests/test_ig_c99/Makefile
index 5e822b3..317d9c6 100644
--- a/lm-c99/tests/test_ig_c99/Makefile
+++ b/lm-c99/tests/test_ig_c99/Makefile
@@ -15,6 +15,7 @@ xml: $(SRC_XML)
 
 $(BUILD_DIR)/*: $(BUILD_DIR) $(SRC) $(COMPILER)
 	YML_PATH=$(YML_PATH) yml2proc -y $(COMPILER) $(SRC)
+	diff $(BUILD_DIR)/idl_core.h $(BUILD_DIR)/idl_core.h.target
 	diff $(BUILD_DIR)/test_api.h $(BUILD_DIR)/test_api.h.target
 
 $(BUILD_DIR):