diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index a4f2a145f7..62d929c837 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2017-09-05  Tom Tromey  <tom@tromey.com>
+
+	PR gdb/22010:
+	* gdb.base/sizeof.exp (check_valueof): Cast char constant to int.
+
 2017-09-06  Thomas Preud'homme  <thomas.preudhomme@arm.com>
 
 	* gdb.arch/thumb2-it.exp: Cast call to assembly defined function.
diff --git a/gdb/testsuite/gdb.base/sizeof.exp b/gdb/testsuite/gdb.base/sizeof.exp
index 5d8940715c..13d36f8a93 100644
--- a/gdb/testsuite/gdb.base/sizeof.exp
+++ b/gdb/testsuite/gdb.base/sizeof.exp
@@ -86,7 +86,7 @@ proc check_valueof { exp val } {
 
 # Check that GDB and the target agree over the sign of a character.
 
-set signof_byte [get_integer_valueof "'\\377'" -1]
+set signof_byte [get_integer_valueof "(int) '\\377'" -1]
 set signof_char [get_integer_valueof "(int) (char) -1" -1]
 set signof_signed_char [get_integer_valueof "(int) (signed char) -1" -1]
 set signof_unsigned_char [get_integer_valueof "(int) (unsigned char) -1" -1]