diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6cc35782015..6c2100d65bd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -84,6 +84,7 @@
 * [BUGFIX] Ring: Fix DynamoDB KV CAS not retrying on transactional conditional check failures. `TransactWriteItems` reports condition failures as `TransactionCanceledException` with a `ConditionalCheckFailed` cancellation reason, which was not recognized as retryable, so any concurrent ring update conflict (e.g. many ingesters joining during a rolling update) failed immediately instead of re-reading and retrying. `TransactionConflict` cancellation reasons are also treated as retryable. #7706
 * [BUGFIX] Distributor: Return HTTP 499 (Client Closed Request) instead of 500 when a remote-write or OTLP push is canceled by the client, so client-side cancellations are no longer counted as server-side errors. #7717
 * [BUGFIX] Querier: Fix gRPC `codes.Canceled` errors being mapped to HTTP 500 instead of 499 when a client cancels a query. #7738
+* [BUGFIX] Querier: Fix panic (`index out of range [-1]`) in the active request tracker when truncating a `match[]`/`query` value made entirely of invalid UTF-8 continuation bytes. The backwards scan for a rune boundary now stops at index 0 instead of underflowing. #7729
 
 ## 1.21.1 2026-06-04
 
diff --git a/pkg/util/request_tracker/request_extractor.go b/pkg/util/request_tracker/request_extractor.go
index cbd9f31e8fe..b9e7ff4d0f0 100644
--- a/pkg/util/request_tracker/request_extractor.go
+++ b/pkg/util/request_tracker/request_extractor.go
@@ -83,7 +83,7 @@ func trimStringByBytes(str string, size int) string {
 	bytesStr := []byte(str)
 	trimIndex := len(bytesStr)
 	if size < len(bytesStr) {
-		for !utf8.RuneStart(bytesStr[size]) {
+		for size > 0 && !utf8.RuneStart(bytesStr[size]) {
 			size--
 		}
 		trimIndex = size
diff --git a/pkg/util/request_tracker/request_tracker_test.go b/pkg/util/request_tracker/request_tracker_test.go
index 13ef0a802cd..5cdc6fed2b5 100644
--- a/pkg/util/request_tracker/request_tracker_test.go
+++ b/pkg/util/request_tracker/request_tracker_test.go
@@ -161,6 +161,20 @@ func TestTrimForJsonMarshalMultiByteUTF8(t *testing.T) {
 	}
 }
 
+// TestTrimForJsonMarshalInvalidUTF8 reproduces a panic where a string made
+// entirely of UTF-8 continuation bytes (0x80-0xBF, no valid rune-start byte)
+// caused the backwards scan for a rune boundary to underflow past index 0
+// and index the byte slice with a negative index.
+func TestTrimForJsonMarshalInvalidUTF8(t *testing.T) {
+	invalid := strings.Repeat("\x80", 1200)
+
+	require.NotPanics(t, func() {
+		out := trimForJsonMarshal(invalid, 800)
+		assert.True(t, utf8.ValidString(out), "result should be valid UTF-8")
+		assert.Equal(t, "", out)
+	})
+}
+
 // TestGenerateJSONEntryWithTruncatedFieldNegativeSize reproduces the request
 // tracker panic where a multi-byte UTF-8 field had to be truncated to a
 // negative remaining size because the rest of the entry already consumed the