diff --git a/tools/gha_osx.sh b/tools/gha_osx.sh
index 0c702877e..0b2418d25 100644
--- a/tools/gha_osx.sh
+++ b/tools/gha_osx.sh
@@ -21,7 +21,7 @@ mkdir build
 cd build
 
 # GCC build.
-CXX=clang++ CC=clang cmake ../ -DCMAKE_PREFIX_PATH=$deps_dir -DCMAKE_BUILD_TYPE=Debug -DBoost_NO_BOOST_CMAKE=ON -DPAGMO_BUILD_TESTS=yes -DPAGMO_WITH_EIGEN3=yes -DPAGMO_WITH_NLOPT=yes -DPAGMO_WITH_IPOPT=yes -DPAGMO_ENABLE_IPO=yes
+CXXFLAGS="-fno-assume-unique-vtables" CXX=clang++ CC=clang cmake ../ -DCMAKE_PREFIX_PATH=$deps_dir -DCMAKE_BUILD_TYPE=Debug -DBoost_NO_BOOST_CMAKE=ON -DPAGMO_BUILD_TESTS=yes -DPAGMO_WITH_EIGEN3=yes -DPAGMO_WITH_NLOPT=yes -DPAGMO_WITH_IPOPT=yes -DPAGMO_ENABLE_IPO=yes
 make -j2 VERBOSE=1
 ctest -V -j4