(gdb) file ./python
Reading symbols from ./python...
warning: File "/usr/src/cpython/python-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
To enable execution of this file add
	add-auto-load-safe-path /usr/src/cpython/python-gdb.py
line to your configuration file "/root/.gdbinit".
To completely disable this security protection add
	set auto-load safe-path /
line to your configuration file "/root/.gdbinit".
For more information about this security protection see the
"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:
	info "(gdb)Auto-loading safe path"
(gdb) break tok_get_fstring_mode
Breakpoint 1 at 0x17119c: file Parser/tokenizer.c, line 2442.
(gdb) r t.py
Starting program: /usr/src/cpython/python t.py
warning: Error disabling address space randomization: Operation not permitted
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/aarch64-linux-gnu/libthread_db.so.1".

Breakpoint 1, tok_get_fstring_mode (tok=0xaaab0ecfec60, current_tok=0xaaab0ecff7d0, token=0xffffc95f8278) at Parser/tokenizer.c:2442
2442	{
(gdb) c
Continuing.

Breakpoint 1, tok_get_fstring_mode (tok=0xaaab0ecfec60, current_tok=0xaaab0ecff7d0, token=0xffffc95f8278) at Parser/tokenizer.c:2442
2442	{
(gdb) p tok->cur
$1 = 0xffff883fc1e3 "\""
(gdb) p tok->buf
$2 = 0xffff883fc1e0 "f\"a\""
(gdb) n
2448	    tok->start = tok->cur;
(gdb)
2449	    tok->first_lineno = tok->lineno;
(gdb)
2450	    tok->starting_col_offset = tok->col_offset;
(gdb)
2454	    char start_char = tok_nextc(tok);
(gdb)
2455	    char peek1 = tok_nextc(tok);
(gdb) p start_char
$3 = 34 '"'
(gdb) s
tok_nextc (tok=0xaaab0ecfec60) at Parser/tokenizer.c:1169
1169	{
(gdb) n
1172	        if (tok->cur != tok->inp) {
(gdb)
1176	        if (tok->done != E_OK) {
(gdb)
1179	        if (tok->fp == NULL) {
(gdb)
1180	            rc = tok_underflow_string(tok);
(gdb) s
tok_underflow_string (tok=0xaaab0ecfec60) at Parser/tokenizer.c:965
965	tok_underflow_string(struct tok_state *tok) {
(gdb) list
960	    } while (tok->inp[-1] != '\n');
961	    return 1;
962	}
963
964	static int
965	tok_underflow_string(struct tok_state *tok) {
966	    char *end = strchr(tok->inp, '\n');
967	    if (end != NULL) {
968	        end++;
969	    }
(gdb)
970	    else {
971	        end = strchr(tok->inp, '\0');
972	        if (end == tok->inp) {
973	            tok->done = E_EOF;
974	            return 0;
975	        }
976	    }
977	    if (tok->start == NULL) {
978	        tok->buf = tok->cur;
979	    }
(gdb) n
966	    char *end = strchr(tok->inp, '\n');
(gdb)
967	    if (end != NULL) {
(gdb)
971	        end = strchr(tok->inp, '\0');
(gdb)
972	        if (end == tok->inp) {
(gdb)
973	            tok->done = E_EOF;
(gdb)
974	            return 0;
(gdb)
tok_nextc (tok=0xaaab0ecfec60) at Parser/tokenizer.c:1189
1189	        if (tok->debug) {
(gdb) list
1184	        }
1185	        else {
1186	            rc = tok_underflow_file(tok);
1187	        }
1188	#if defined(Py_DEBUG)
1189	        if (tok->debug) {
1190	            fprintf(stderr, "line[%d] = ", tok->lineno);
1191	            print_escape(stderr, tok->cur, tok->inp - tok->cur);
1192	            fprintf(stderr, "  tok->done = %d\n", tok->done);
1193	        }
(gdb)
1194	#endif
1195	        if (!rc) {
1196	            tok->cur = tok->inp;
1197	            return EOF;
1198	        }
1199	        tok->line_start = tok->cur;
1200
1201	        if (contains_null_bytes(tok->line_start, tok->inp - tok->line_start)) {
1202	            syntaxerror(tok, "source code cannot contain null bytes");
1203	            tok->cur = tok->inp;
(gdb) n
1195	        if (!rc) {
(gdb)
1196	            tok->cur = tok->inp;
(gdb)
1197	            return EOF;
(gdb)
tok_get_fstring_mode (tok=0xaaab0ecfec60, current_tok=0xaaab0ecff7d0, token=0xffffc95f8278) at Parser/tokenizer.c:2456
2456	    tok_backup(tok, peek1);
(gdb) p peek1
$4 = 255 '\377'

diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index d88d737860..34f291cf89 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -2452,8 +2452,14 @@ tok_get_fstring_mode(struct tok_state *tok, tokenizer_mode* current_tok, struct
     // If we start with a bracket, we defer to the normal mode as there is nothing for us to tokenize
     // before it.
     char start_char = tok_nextc(tok);
-    char peek1 = tok_nextc(tok);
-    tok_backup(tok, peek1);
+    char peek1;
+    if (start_char == '{' || start_char == '}') {
+        peek1 = tok_nextc(tok);
+        tok_backup(tok, peek1);
+    }
+    else {
+        peek1 = '0';
+    }
     tok_backup(tok, start_char);
 
     if ((start_char == '{' && peek1 != '{') || (start_char == '}' && peek1 != '}')) {