diff --git a/backend/go/whisper/gowhisper.go b/backend/go/whisper/gowhisper.go
index bf7cb7a45..56d794f0f 100644
--- a/backend/go/whisper/gowhisper.go
+++ b/backend/go/whisper/gowhisper.go
@@ -139,7 +139,10 @@ func (w *Whisper) AudioTranscription(opts *pb.TranscriptRequest) (pb.TranscriptR
 		// segment start/end conversion factor taken from https://github.com/ggml-org/whisper.cpp/blob/master/examples/cli/cli.cpp#L895
 		s := CppGetSegmentStart(i) * (10000000)
 		t := CppGetSegmentEnd(i) * (10000000)
-		txt := strings.Clone(CppGetSegmentText(i))
+		// whisper.cpp can emit bytes that aren't valid UTF-8 (e.g. a multibyte
+		// codepoint split across token boundaries); protobuf string fields
+		// reject those at marshal time. Scrub before the value escapes cgo.
+		txt := strings.ToValidUTF8(strings.Clone(CppGetSegmentText(i)), "�")
 		tokens := make([]int32, CppNTokens(i))
 
 		if opts.Diarize && CppGetSegmentSpeakerTurnNext(i) {