diff --git a/Dockerfile b/Dockerfile
index 934a02a..d524aa9 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -140,6 +140,7 @@ RUN git clone https://github.com/dvorak0/gtsam.git -b yzf/add_smart_factor_pytho
     && mkdir build && cd build \
     && cmake -DCMAKE_BUILD_TYPE=Release -DGTSAM_BUILD_PYTHON=ON -DGTSAM_THROW_CHEIRALITY_EXCEPTION=OFF .. \
     && make -j2
+ENV PYTHONPATH="${PYTHONPATH:-}"
 ENV PYTHONPATH="/3rdparty/gtsam/build/python:${PYTHONPATH}"
 
 # clean