diff --git a/vlib/builtin/int.v b/vlib/builtin/int.v
index d64ee16a02..44369a8ed4 100644
--- a/vlib/builtin/int.v
+++ b/vlib/builtin/int.v
@@ -266,6 +266,7 @@ pub fn (n int) hex1() string {
 	len := if n >= 0 { n.str().len + 3 } else { 11 }
 	hex := malloc(len) // 0x + \n
 	count := C.sprintf((hex), '0x%x', n)
+    hex[count] = 0
 	return tos(hex, count)
 }
 */
@@ -282,10 +283,7 @@ fn u64_to_hex(nn u64, len byte) string {
 		buf[i] = x
 		n = n >> 4
 	}
-	return string{
-		str: unsafe { memdup(buf, len + 1) }
-		len: len
-	}
+	return unsafe { tos(memdup(buf, len + 1), len) }
 }
 
 // u64_to_hex_no_leading_zeros converts the number `nn` to hexadecimal `string`.
@@ -305,10 +303,7 @@ fn u64_to_hex_no_leading_zeros(nn u64, len byte) string {
 		}
 	}
 	res_len := len - i
-	return string{
-		str: unsafe { memdup(&buf[i], res_len + 1) }
-		len: res_len
-	}
+	return unsafe { tos(memdup(&buf[i], res_len + 1), res_len) }
 }
 
 // hex returns the value of the `byte` as a hexadecimal `string`.
diff --git a/vlib/builtin/rune.v b/vlib/builtin/rune.v
index 8cbb45acc8..325cba6e36 100644
--- a/vlib/builtin/rune.v
+++ b/vlib/builtin/rune.v
@@ -35,8 +35,8 @@ pub fn (c byte) is_capital() bool {
 
 pub fn (b []byte) clone() []byte {
 	mut res := []byte{len: b.len}
-	//mut res := make([]byte, {repeat:b.len})
-	for i in 0..b.len {
+	// mut res := make([]byte, {repeat:b.len})
+	for i in 0 .. b.len {
 		res[i] = b[i]
 	}
 	return res
@@ -49,8 +49,10 @@ pub fn (b []byte) bytestr() string {
 
 // TODO copy pasted from builder.v
 fn bytes2string(b []byte) string {
-	mut copy := b.clone()
-	copy << `\0`
-	res := unsafe { tos(copy.data, copy.len-1) }
-	return res
+	unsafe {
+		buf := malloc(b.len + 1)
+		C.memcpy(buf, b.data, b.len)
+		buf[b.len] = 0
+		return tos(buf, b.len)
+	}
 }
diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index c3b4aacd22..515b202133 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -877,6 +877,7 @@ pub fn (s string) to_lower() string {
 		for i in 0 .. s.len {
 			b[i] = byte(C.tolower(s.str[i]))
 		}
+		b[s.len] = 0
 		return tos(b, s.len)
 	}
 }
@@ -900,6 +901,7 @@ pub fn (s string) to_upper() string {
 		for i in 0 .. s.len {
 			b[i] = byte(C.toupper(s.str[i]))
 		}
+		b[s.len] = 0
 		return tos(b, s.len)
 	}
 }
diff --git a/vlib/builtin/utf8.v b/vlib/builtin/utf8.v
index f886060aa5..0d26f9cb56 100644
--- a/vlib/builtin/utf8.v
+++ b/vlib/builtin/utf8.v
@@ -25,17 +25,20 @@ pub fn utf32_to_str_no_malloc(code u32, buf voidptr) string {
 		if icode <= 127 {
 			// 0x7F
 			buffer[0] = byte(icode)
+			buffer[1] = 0
 			res = tos(buffer, 1)
 		} else if icode <= 2047 {
 			// 0x7FF
 			buffer[0] = 192 | byte(icode >> 6) // 0xC0 - 110xxxxx
 			buffer[1] = 128 | byte(icode & 63) // 0x80 - 0x3F - 10xxxxxx
+			buffer[2] = 0
 			res = tos(buffer, 2)
 		} else if icode <= 65535 {
 			// 0xFFFF
 			buffer[0] = 224 | byte(icode >> 12) // 0xE0 - 1110xxxx
 			buffer[1] = 128 | (byte(icode >> 6) & 63) // 0x80 - 0x3F - 10xxxxxx
 			buffer[2] = 128 | byte(icode & 63) // 0x80 - 0x3F - 10xxxxxx
+			buffer[3] = 0
 			res = tos(buffer, 3)
 		}
 		// 0x10FFFF
@@ -44,6 +47,7 @@ pub fn utf32_to_str_no_malloc(code u32, buf voidptr) string {
 			buffer[1] = 128 | (byte(icode >> 12) & 63) // 0x80 - 0x3F - 10xxxxxx
 			buffer[2] = 128 | (byte(icode >> 6) & 63) // 0x80 - 0x3F - 10xxxxxx
 			buffer[3] = 128 | byte(icode & 63) // 0x80 - 0x3F - 10xxxxxx
+			buffer[4] = 0
 			res = tos(buffer, 4)
 		}
 	}