diff --git a/cmake/MOAB_PullAndMake.cmake b/cmake/MOAB_PullAndMake.cmake
index 57ca0d0b1..4809bd0df 100644
--- a/cmake/MOAB_PullAndMake.cmake
+++ b/cmake/MOAB_PullAndMake.cmake
@@ -91,7 +91,7 @@ IF (PULL_INSTALL_MOAB)
   ENDIF()
 
   IF (DEFINED ${EIGEN3_DIR})
-    OPTION_FLAG = "-DEIGEN3_DIR=${EIGEN3_DIR}"
+    SET(OPTION_FLAG "-DEIGEN3_DIR=${EIGEN3_DIR}")
   ENDIF()
 
 ENDIF ()