diff --git a/src/log.c b/src/log.c
index 6c6efed..37c2dbd 100644
--- a/src/log.c
+++ b/src/log.c
@@ -1801,9 +1801,9 @@
 
 			case LOG_FMT_EXPR: // sample expression, may be request or response
 				key = NULL;
-				if (tmp->options & LOG_OPT_REQ_CAP)
+				if (tmp->options & LOG_OPT_REQ_CAP && s)
 					key = sample_fetch_as_type(be, sess, s, SMP_OPT_DIR_REQ|SMP_OPT_FINAL, tmp->expr, SMP_T_STR);
-				if (!key && (tmp->options & LOG_OPT_RES_CAP))
+				if (!key && (tmp->options & LOG_OPT_RES_CAP) && s)
 					key = sample_fetch_as_type(be, sess, s, SMP_OPT_DIR_RES|SMP_OPT_FINAL, tmp->expr, SMP_T_STR);
 				if (tmp->options & LOG_OPT_HTTP)
 					ret = lf_encode_chunk(tmplog, dst + maxsize,