diff --git a/src/syntax.c b/src/syntax.c
index e37dacb..ac80ab4 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -8999,6 +8999,8 @@
 	    n = HL_TABLE()[id - 1].sg_cterm_fg - 1;
 	else
 	    n = HL_TABLE()[id - 1].sg_cterm_bg - 1;
+	if (n < 0)
+	    return NULL;
 	sprintf((char *)name, "%d", n);
 	return name;
     }