diff --git a/src/bonsai/Makefile b/src/bonsai/Makefile
index 5bc39cbd84..b3b1ee49a3 100644
--- a/src/bonsai/Makefile
+++ b/src/bonsai/Makefile
@@ -192,6 +192,8 @@ else ifeq ($(PLATFORM), macos)
 else
 	cd build && . env/$(VENV_ACTIVATE) && $(PIP) download pyradiance $(PYPI_PLATFORM) --python-version $(PYPI_VERSION) --implementation $(PYPI_IMP) --only-binary=:all: --dest=./wheels
 endif
+	# Required by ifctester web ui.
+	cd build && . env/$(VENV_ACTIVATE) && $(PIP) download flask --dest=./wheels
 
 	# jQuery is required for web UI functionality
 	cd build/bonsai/bim/data/webui/static/js/ && wget https://cdn.jsdelivr.net/npm/jquery@3.7.1/dist/jquery.min.js