diff --git a/src/spellfile.c b/src/spellfile.c
index 5b34298..370e80b 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -2018,7 +2018,7 @@
 // In the postponed prefixes tree wn_flags is used to store the WFP_ flags,
 // but it must be negative to indicate the prefix tree to tree_add_word().
 // Use a negative number with the lower 8 bits zero.
-#define PFX_FLAGS	-256
+#define PFX_FLAGS	(-256)
 
 // flags for "condit" argument of store_aff_word()
 #define CONDIT_COMB	1	// affix must combine