diff --git a/lib/regex/regex.go b/lib/regex/regex.go
index 98e0e5e..ebb5e61 100644
--- a/lib/regex/regex.go
+++ b/lib/regex/regex.go
@@ -605,9 +605,14 @@ func (m *Match) Hash() (uint32, error) { return 0, fmt.Errorf("unhashable type:
 func (m *Match) groupIndex(v starlark.Value) (int, error) {
 	switch g := v.(type) {
 	case starlark.Int:
-		i, _ := g.Int64()
-		if i < 0 || int(i) > m.p.search.NumSubexp() {
-			return 0, fmt.Errorf("no such group: %d", i)
+		// Int64 reports ok=false when the index overflows int64; the
+		// discarded ok used to leave i==0, silently selecting group 0 (the
+		// whole match) for a huge index instead of erroring. Compare as
+		// int64 to avoid truncating on the way to int, and format the
+		// original value (an overflowing index prints as 0 via %d).
+		i, ok := g.Int64()
+		if !ok || i < 0 || i > int64(m.p.search.NumSubexp()) {
+			return 0, fmt.Errorf("no such group: %s", g.String())
 		}
 		return int(i), nil
 	case starlark.String:
diff --git a/lib/regex/regex_test.go b/lib/regex/regex_test.go
index 770c800..c0edf98 100644
--- a/lib/regex/regex_test.go
+++ b/lib/regex/regex_test.go
@@ -233,6 +233,21 @@ func TestLoadModule_Regex(t *testing.T) {
 			`),
 			wantErr: `no such group: 5`,
 		},
+		{
+			// a group index too large for int64 used to be silently truncated
+			// to 0 (the whole match) instead of erroring; group/start/end/span
+			// all route through groupIndex and must reject it loudly.
+			name: `error: group index overflows int64`,
+			script: itn.HereDoc(`
+				load('regex', 'search')
+				m = search('(a)', 'a')
+				assert.eq(m.group(1), 'a')
+				assert.fails(lambda: m.group(1 << 70), 'no such group')
+				assert.fails(lambda: m.start(1 << 70), 'no such group')
+				assert.fails(lambda: m.end(1 << 70), 'no such group')
+				assert.fails(lambda: m.span(1 << 70), 'no such group')
+			`),
+		},
 		{
 			name: `error: bad repl type`,
 			script: itn.HereDoc(`