diff --git a/cmp/src/ref.c b/cmp/src/ref.c
index 26ecc21..eccf470 100644
--- a/cmp/src/ref.c
+++ b/cmp/src/ref.c
@@ -34,6 +34,7 @@ static const u8 values[256] = {
 void SUFFIX(encode_seq, COMPILER)
 (u8 const* restrict input, usize len, u8* restrict output)
 {
+	len -= len % 3;
 	for (; len; len -= 3) {
 		*output++ = symbols[(u8)(input[0] >> 2)];
 		*output++ = symbols[(u8)(input[0] << 4 | input[1] >> 4)];
@@ -46,6 +47,7 @@ void SUFFIX(encode_seq, COMPILER)
 void SUFFIX(encode_par, COMPILER)
 (u8 const* restrict input, usize len, u8* restrict output)
 {
+	len -= len % 3;
 	for (; len; len -= 3) {
 		u64 x = (u64)input[0] << 16 | (u64)input[1] << 8 | input[2];
 		output[0] = symbols[(u8)(x >> 18)];
@@ -60,6 +62,7 @@ void SUFFIX(encode_par, COMPILER)
 i32 SUFFIX(decode_seq, COMPILER)
 (u8 const* restrict input, usize len, u8* restrict output)
 {
+	len -= len % 4;
 	for (; len; len -= 4) {
 		if (values[input[0]] >= 64) return -1;
 		if (values[input[1]] >= 64) return -1;
@@ -76,6 +79,7 @@ i32 SUFFIX(decode_seq, COMPILER)
 i32 SUFFIX(decode_par, COMPILER)
 (u8 const* restrict input, usize len, u8* restrict output)
 {
+	len -= len % 4;
 	for (; len; len -= 4) {
 		if (values[input[0]] & values[input[1]] & values[input[2]]
 		    & values[input[3]] & 0xc0) {
diff --git a/cmp/tests/lib.rs b/cmp/tests/lib.rs
index 2c00b79..3304df0 100644
--- a/cmp/tests/lib.rs
+++ b/cmp/tests/lib.rs
@@ -54,3 +54,39 @@ fn difference() {
     assert_eq!(BASE64.decode(x).unwrap(), vec![0, 0]);
     assert_eq!(BASE64_STANDARD.decode(x).err().unwrap(), InvalidByte(2, b'='));
 }
+
+#[test]
+fn partial_block() {
+    let encode = &[
+        (b"f" as &[u8], b"" as &[u8]),
+        (b"fo" as &[u8], b"" as &[u8]),
+        (b"foob" as &[u8], b"Zm9v" as &[u8]),
+        (b"fooba" as &[u8], b"Zm9v" as &[u8]),
+    ];
+    for &(ref i, ref o) in encode {
+        let mut r = vec![0u8; o.len()];
+        cmp::base64_encode_seq_gcc(i, &mut r);
+        assert_eq!(&r, o);
+    }
+    for &(ref i, ref o) in encode {
+        let mut r = vec![0u8; o.len()];
+        cmp::base64_encode_par_gcc(i, &mut r);
+        assert_eq!(&r, o);
+    }
+    let decode = &[
+        (b"Z" as &[u8], b"" as &[u8]),
+        (b"Zm9" as &[u8], b"" as &[u8]),
+        (b"Zm9vY" as &[u8], b"foo" as &[u8]),
+        (b"Zm9vYmF" as &[u8], b"foo" as &[u8]),
+    ];
+    for &(ref i, ref o) in decode {
+        let mut r = vec![0u8; o.len()];
+        assert_eq!(cmp::base64_decode_seq_gcc(i, &mut r), Ok(()));
+        assert_eq!(&r, o);
+    }
+    for &(ref i, ref o) in decode {
+        let mut r = vec![0u8; o.len()];
+        assert_eq!(cmp::base64_decode_par_gcc(i, &mut r), Ok(()));
+        assert_eq!(&r, o);
+    }
+}