diff --git a/src/classes/CMakeLists.txt b/src/classes/CMakeLists.txt
index ad287bae..ff5e777d 100644
--- a/src/classes/CMakeLists.txt
+++ b/src/classes/CMakeLists.txt
@@ -59,6 +59,7 @@ root_generate_dictionary(
   OPTIONS 
     -I${NONLOCAL_INC} 
     -I${MUSRFIT_INC}
+    -I${FFTW3_INCLUDE_DIR}
     -I${CMAKE_CURRENT_SOURCE_DIR}
     -inlineInputHeader
   LINKDEF ${MUSRFIT_INC}/PRgeHandlerLinkDef.h