diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 47e4d7c509..79c1208bd5 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-07-03  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/charset.c (init_string): Change all char parameters to
+	unsigned char parameters.
+
 2020-07-03  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/call-sc.exp (start_scalars_test): Use
diff --git a/gdb/testsuite/gdb.base/charset.c b/gdb/testsuite/gdb.base/charset.c
index ec4927da51..20d548b192 100644
--- a/gdb/testsuite/gdb.base/charset.c
+++ b/gdb/testsuite/gdb.base/charset.c
@@ -77,12 +77,21 @@ short short_array[3];
 int int_array[3];
 long long_array[3];
 
+/* These are unsigned char so we can pass down characters >127 without
+   explicit casts or warnings.  */
+
 void
 init_string (char string[],
-             char x,
-             char alert, char backspace, char form_feed,
-             char line_feed, char carriage_return, char horizontal_tab,
-             char vertical_tab, char cent, char misc_ctrl)
+	     unsigned char x,
+	     unsigned char alert,
+	     unsigned char backspace,
+	     unsigned char form_feed,
+	     unsigned char line_feed,
+	     unsigned char carriage_return,
+	     unsigned char horizontal_tab,
+	     unsigned char vertical_tab,
+	     unsigned char cent,
+	     unsigned char misc_ctrl)
 {
   int i;