diff --git a/exp/textinput/textinput.go b/exp/textinput/textinput.go
index 44f9a07d0..4d862c5df 100644
--- a/exp/textinput/textinput.go
+++ b/exp/textinput/textinput.go
@@ -19,6 +19,7 @@
 package textinput
 
 import (
+	"fmt"
 	"image"
 	"unicode/utf16"
 	"unicode/utf8"
@@ -65,27 +66,49 @@ func start(bounds image.Rectangle) (states <-chan textInputState, close func())
 }
 
 func convertUTF16CountToByteCount(text string, c int) int {
+	if !utf8.ValidString(text) {
+		return -1
+	}
 	if c == 0 {
 		return 0
 	}
 	var utf16Len int
 	for idx, r := range text {
-		utf16Len += utf16.RuneLen(r)
+		l16 := utf16.RuneLen(r)
+		if l16 < 0 {
+			panic(fmt.Sprintf("textinput: invalid rune: %c", r))
+		}
+		utf16Len += l16
 		if utf16Len >= c {
-			return idx + utf8.RuneLen(r)
+			l8 := utf8.RuneLen(r)
+			if l8 < 0 {
+				panic(fmt.Sprintf("textinput: invalid rune: %c", r))
+			}
+			return idx + l8
 		}
 	}
 	return -1
 }
 
 func convertByteCountToUTF16Count(text string, c int) int {
+	if !utf8.ValidString(text) {
+		return -1
+	}
 	if c == 0 {
 		return 0
 	}
 	var utf16Len int
 	for idx, r := range text {
-		utf16Len += utf16.RuneLen(r)
-		if idx+utf8.RuneLen(r) >= c {
+		l16 := utf16.RuneLen(r)
+		if l16 < 0 {
+			panic(fmt.Sprintf("textinput: invalid rune length for rune %c", r))
+		}
+		utf16Len += l16
+		l8 := utf8.RuneLen(r)
+		if l8 < 0 {
+			panic(fmt.Sprintf("textinput: invalid rune length for rune %c", r))
+		}
+		if idx+l8 >= c {
 			return utf16Len
 		}
 	}
diff --git a/exp/textinput/textinput_test.go b/exp/textinput/textinput_test.go
index 1d82957b5..d45ca2b1e 100644
--- a/exp/textinput/textinput_test.go
+++ b/exp/textinput/textinput_test.go
@@ -42,6 +42,10 @@ func TestConvertUTF16CountToByteCount(t *testing.T) {
 		{"寿司🍣食べたい", 4, 10},
 		{"寿司🍣食べたい", 5, 13},
 		{"寿司🍣食べたい", 100, -1},
+		{"\xff\xff\xff\xff", 0, -1},
+		{"\xff\xff\xff\xff", 1, -1},
+		{"\xff\xff\xff\xff", 2, -1},
+		{"\xff\xff\xff\xff", 100, -1},
 	}
 	for _, tc := range testCases {
 		if got := textinput.ConvertUTF16CountToByteCount(tc.text, tc.c); got != tc.want {
@@ -72,6 +76,10 @@ func TestConvertByteCountToUTF16Count(t *testing.T) {
 		{"寿司🍣食べたい", 10, 4},
 		{"寿司🍣食べたい", 13, 5},
 		{"寿司🍣食べたい", 100, -1},
+		{"\xff\xff\xff\xff", 0, -1},
+		{"\xff\xff\xff\xff", 3, -1},
+		{"\xff\xff\xff\xff", 6, -1},
+		{"\xff\xff\xff\xff", 100, -1},
 	}
 	for _, tc := range testCases {
 		if got := textinput.ConvertByteCountToUTF16Count(tc.text, tc.c); got != tc.want {
diff --git a/text/v2/gox.go b/text/v2/gox.go
index 1db7a87dc..f1375ebe8 100644
--- a/text/v2/gox.go
+++ b/text/v2/gox.go
@@ -172,6 +172,10 @@ func (g *GoXFace) appendGlyphsForLine(glyphs []Glyph, line string, indexOffset i
 		// Adjust the position to the integers.
 		// The current glyph images assume that they are rendered on integer positions so far.
 		size := utf8.RuneLen(r)
+		if size < 0 {
+			// Treat an error as 1, following DecodeRuneInString.
+			size = 1
+		}
 
 		// Append a glyph even if img is nil.
 		// This is necessary to return index information for control characters.
diff --git a/text/v2/multi.go b/text/v2/multi.go
index 361d93a3d..6c8cedba6 100644
--- a/text/v2/multi.go
+++ b/text/v2/multi.go
@@ -168,7 +168,6 @@ func (m *MultiFace) splitText(text string) iter.Seq[textChunk] {
 		var chunk textChunk
 		for _, r := range text {
 			fi := -1
-			l := utf8.RuneLen(r)
 			for i, f := range m.faces {
 				if !f.hasGlyph(r) && i < len(m.faces)-1 {
 					continue
@@ -180,6 +179,12 @@ func (m *MultiFace) splitText(text string) iter.Seq[textChunk] {
 				panic("text: a face was not selected correctly")
 			}
 
+			l := utf8.RuneLen(r)
+			if l < 0 {
+				// Treat an error as 1, following DecodeRuneInString.
+				l = 1
+			}
+
 			var s int
 			if chunk != (textChunk{}) {
 				if chunk.faceIndex == fi {