diff --git a/Misc/NEWS b/Misc/NEWS
index ae4c06b909a..a76b99557c6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -600,6 +600,8 @@ Extension Modules
 Build
 -----
 
+- Issue #10055: Make json C89-compliant in UCS4 mode.
+
 - Issue #6244: Allow detect_tkinter to look for Tcl/Tk 8.6.
 
 - Issue #1633863: Don't ignore $CC under AIX.
diff --git a/Modules/_json.c b/Modules/_json.c
index 2e395255f0a..4da5e74dc26 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -430,8 +430,8 @@ scanstring_unicode(PyObject *pystr, Py_ssize_t end, int strict, Py_ssize_t *next
                 end += 6;
                 /* Decode 4 hex digits */
                 for (; next < end; next++) {
-                    c2 <<= 4;
                     Py_UNICODE digit = buf[next];
+                    c2 <<= 4;
                     switch (digit) {
                         case '0': case '1': case '2': case '3': case '4':
                         case '5': case '6': case '7': case '8': case '9':