diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7ccd9b3..fb09c80 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -24,6 +24,7 @@ endif()
 set(EIGEN_VERSION 3.3)
 find_package(Eigen3 ${EIGEN_VERSION} QUIET)
 if(NOT EIGEN3_FOUND)
+  include(FetchContent)
   set(BUILD_TESTING OFF CACHE INTERNAL "")
   FetchContent_Declare(eigen
     GIT_REPOSITORY  https://gitlab.com/libeigen/eigen.git