diff --git a/src/net/textproto/reader.go b/src/net/textproto/reader.go
index d375340121fa0b..668c06c24c1372 100644
--- a/src/net/textproto/reader.go
+++ b/src/net/textproto/reader.go
@@ -110,11 +110,12 @@ func trim(s []byte) []byte {
 	for i < len(s) && (s[i] == ' ' || s[i] == '\t') {
 		i++
 	}
-	n := len(s)
-	for n > i && (s[n-1] == ' ' || s[n-1] == '\t') {
+	s = s[i:]
+	n := len(s) - 1
+	for n >= 0 && (s[n] == ' ' || s[n] == '\t') {
 		n--
 	}
-	return s[i:n]
+	return s[:n+1]
 }
 
 // ReadContinuedLineBytes is like [Reader.ReadContinuedLine] but