diff --git a/src/encoding/base64/base64.go b/src/encoding/base64/base64.go
index b15754ee933..be69271d195 100644
--- a/src/encoding/base64/base64.go
+++ b/src/encoding/base64/base64.go
@@ -35,13 +35,19 @@ const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz012345678
 const encodeURL = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
 
 // NewEncoding returns a new padded Encoding defined by the given alphabet,
-// which must be a 64-byte string.
+// which must be a 64-byte string that does not contains the padding character
+// or CR / LF ('\r', '\n').
 // The resulting Encoding uses the default padding character ('='),
 // which may be changed or disabled via WithPadding.
 func NewEncoding(encoder string) *Encoding {
 	if len(encoder) != 64 {
 		panic("encoding alphabet is not 64-bytes long")
 	}
+	for i := 0; i < len(encoder); i++ {
+		if encoder[i] == '\n' || encoder[i] == '\r' {
+			panic("encoding alphabet contains newline character")
+		}
+	}
 
 	e := new(Encoding)
 	e.padChar = StdPadding
@@ -58,7 +64,20 @@ func NewEncoding(encoder string) *Encoding {
 
 // WithPadding creates a new encoding identical to enc except
 // with a specified padding character, or NoPadding to disable padding.
+// The padding character must not be '\r' or '\n', must not
+// be contained in the encoding's alphabet and must be a rune equal or
+// below '\xff'.
 func (enc Encoding) WithPadding(padding rune) *Encoding {
+	if padding == '\r' || padding == '\n' || padding > 0xff {
+		panic("invalid padding")
+	}
+
+	for i := 0; i < len(enc.encode); i++ {
+		if rune(enc.encode[i]) == padding {
+			panic("padding contained in alphabet")
+		}
+	}
+
 	enc.padChar = padding
 	return &enc
 }