diff --git a/Makefile.am b/Makefile.am
index b9813f3aba..84a4589292 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1084,7 +1084,7 @@ libnm_core_libnm_core_la_CPPFLAGS = \
 
 if WITH_JSON_VALIDATION
 libnm_core_libnm_core_la_CPPFLAGS += $(JANSSON_CFLAGS)
-libnm_core_libnm_core_la_CFLAGS = -fcommon
+libnm_core_libnm_core_la_CPPFLAGS += -fcommon
 endif
 
 libnm_core_libnm_core_la_SOURCES = \