diff --git a/ChangeLog b/ChangeLog
index c6ad16f30..aab79bd5f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,6 +7,7 @@
 	(as OSX has done) and mark the numbered macros as obsolete.
 	* Source/NSMessagePort.m:
 	* Source/NSSocketPort.m: Log if we fail to tuirn on keepalive
+	* Source/NSURL.m: fix possible nul pointer dereference
 
 2018-02-05  Richard Frith-Macdonald <rfm@gnu.org>
 
diff --git a/Source/NSURL.m b/Source/NSURL.m
index 84954c0e0..3a8f8b044 100644
--- a/Source/NSURL.m
+++ b/Source/NSURL.m
@@ -347,14 +347,18 @@ static char *buildURL(parsedURL *base, parsedURL *rel, BOOL standardize)
   else
     {
       char	*start = base->path;
-      char	*end = strrchr(start, '/');
 
-      if (end != 0)
-	{
-	  *tmp++ = '/';
-	  memcpy(tmp, start, end - start);
-	  tmp += (end - start);
-	}
+      if (start != 0)
+        {
+          char	*end = strrchr(start, '/');
+
+          if (end != 0)
+            {
+              *tmp++ = '/';
+              memcpy(tmp, start, end - start);
+              tmp += (end - start);
+            }
+        }
       *tmp++ = '/';
       l = strlen(rpath);
       memcpy(tmp, rpath, l);