diff --git a/treesource.c b/treesource.c
index 3a87b9f..a55d1d1 100644
--- a/treesource.c
+++ b/treesource.c
@@ -178,7 +178,7 @@ static void write_propval_bytes(FILE *f, struct data val)
 			m = m->next;
 		}
 
-		fprintf(f, "%02hhx", *bp++);
+		fprintf(f, "%02hhx", (unsigned char)(*bp++));
 		if ((const void *)bp >= propend)
 			break;
 		fprintf(f, " ");