diff --git a/build/makefile b/build/makefile
index df7c65d..d5fa593 100644
--- a/build/makefile
+++ b/build/makefile
@@ -68,7 +68,9 @@ FFLAGS  += ${CPPPREFIX}-DNDIMS=${NDIMS}
 
 # output data format
 #
-FFLAGS  += ${CPPPREFIX}-D${OUTPUT}
+ifneq (,$(findstring HDF5,$(OUTPUT)))
+FFLAGS  += ${CPPPREFIX}-DHDF5
+endif
 
 # add module path to compiler options
 #