diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index b2743646511..a7ee02cb55c 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2021-05-17  Tom Tromey  <tromey@adacore.com>
+
+	* source.c (add_path): Check 'p' before using 'p[-1]'.
+
 2021-05-17  Tom Tromey  <tromey@adacore.com>
 
 	* dwarf2/read.h (struct dwarf2_per_cu_data_deleter: New.
diff --git a/gdb/source.c b/gdb/source.c
index 6fc27ae72f7..b6dab6eb236 100644
--- a/gdb/source.c
+++ b/gdb/source.c
@@ -537,6 +537,7 @@ add_path (const char *dirname, char **which_path, int parse_separators)
       /* On MS-DOS and MS-Windows, h:\ is different from h: */
 	     && !(p == name + 3 && name[1] == ':')		/* "d:/" */
 #endif
+	     && p > name
 	     && IS_DIR_SEPARATOR (p[-1]))
 	/* Sigh.  "foo/" => "foo" */
 	--p;