diff --git a/src/buffer.c b/src/buffer.c
index fd94a27..8986ac4 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1955,6 +1955,7 @@
     clear_string_option(&buf->b_p_nf);
 #ifdef FEAT_SYN_HL
     clear_string_option(&buf->b_p_syn);
+    clear_string_option(&buf->b_s.b_syn_isk);
 #endif
 #ifdef FEAT_SPELL
     clear_string_option(&buf->b_s.b_p_spc);