diff --git a/src/lsp/mislib.lsp b/src/lsp/mislib.lsp
index 62ee1bb65..dcbeecb96 100644
--- a/src/lsp/mislib.lsp
+++ b/src/lsp/mislib.lsp
@@ -251,11 +251,11 @@ Universal Time UT, which defaults to the current time."
                         #.(encode-universal-time 0 0 0 1 1 2033 0))
                     (- universal-time (encode-universal-time 0 0 0 1 1 year 0) utc-1-1-1970)))))
     #-ecl-min
-    (ffi::c-inline (unix-time) (:unsigned-long) :bool "
+    (ffi::c-inline (unix-time) (:unsigned-long-long) :bool "
 {
         time_t when = (#0);
         struct tm *ltm = localtime(&when);
-        @(return) = ltm->tm_isdst;
+        @(return) = (ltm != NULL) && ltm->tm_isdst;
 }"
                  :one-liner nil)))