diff --git a/contrib/ltree/ltree.h b/contrib/ltree/ltree.h
index c732a37fa2d..edcda22e4b9 100644
--- a/contrib/ltree/ltree.h
+++ b/contrib/ltree/ltree.h
@@ -87,7 +87,7 @@ typedef struct
 #ifndef abs
 #define abs(a)					((a) <	(0) ? -(a) : (a))
 #endif
-#define ISALNUM(x)	( isalnum((unsigned int)(x)) || (x) == '_' )
+#define ISALNUM(x)	( isalnum((unsigned char)(x)) || (x) == '_' )
 
 /* full text query */