diff --git a/setup.c b/setup.c
index a3b76de2bb..346ef2eb2d 100644
--- a/setup.c
+++ b/setup.c
@@ -46,7 +46,7 @@ const char *prefix_filename(const char *pfx, int pfx_len, const char *arg)
 {
 	static char path[PATH_MAX];
 #ifndef WIN32
-	if (!pfx || !*pfx || is_absolute_path(arg))
+	if (!pfx_len || is_absolute_path(arg))
 		return arg;
 	memcpy(path, pfx, pfx_len);
 	strcpy(path + pfx_len, arg);
@@ -55,7 +55,7 @@ const char *prefix_filename(const char *pfx, int pfx_len, const char *arg)
 	/* don't add prefix to absolute paths, but still replace '\' by '/' */
 	if (is_absolute_path(arg))
 		pfx_len = 0;
-	else
+	else if (pfx_len)
 		memcpy(path, pfx, pfx_len);
 	strcpy(path + pfx_len, arg);
 	for (p = path + pfx_len; *p; p++)