diff --git a/sum.go b/sum.go
index bfc2043..26b3bc6 100644
--- a/sum.go
+++ b/sum.go
@@ -17,6 +17,8 @@ import (
 // ErrSumNotSupported is returned when the Sum function code is not implemented
 var ErrSumNotSupported = errors.New("Function not implemented. Complain to lib maintainer.")
 
+var ErrLenTooLarge = errors.New("requested length was too large for digest")
+
 // HashFunc is a hash function that hashes data into digest.
 //
 // The length is the size the digest will be truncated to. While the hash
@@ -53,6 +55,10 @@ func Sum(data []byte, code uint64, length int) (Multihash, error) {
 	if err != nil {
 		return nil, err
 	}
+	if len(d) < length {
+		return nil, ErrLenTooLarge
+	}
+
 	if length >= 0 {
 		d = d[:length]
 	}
diff --git a/sum_test.go b/sum_test.go
index 296be3a..3226b00 100644
--- a/sum_test.go
+++ b/sum_test.go
@@ -21,6 +21,8 @@ var sumTestCases = []SumTestCase{
 	{SHA1, -1, "foo", "11140beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33"},
 	{SHA1, 10, "foo", "110a0beec7b5ea3f0fdbc95d"},
 	{SHA2_256, -1, "foo", "12202c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"},
+	{SHA2_256, 31, "foo", "121f2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7"},
+	{SHA2_256, 32, "foo", "12202c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"},
 	{SHA2_256, 16, "foo", "12102c26b46b68ffc68ff99b453c1d304134"},
 	{SHA2_512, -1, "foo", "1340f7fbba6e0636f890e56fbbf3283e524c6fa3204ae298382d624741d0dc6638326e282c41be5e4254d8820772c5518a2c5a8c0c7f7eda19594a7eb539453e1ed7"},
 	{SHA2_512, 32, "foo", "1320f7fbba6e0636f890e56fbbf3283e524c6fa3204ae298382d624741d0dc663832"},
@@ -172,3 +174,10 @@ func TestRegisterHashFunc(t *testing.T) {
 		}
 	}
 }
+
+func TestTooLargeLength(t *testing.T) {
+	_, err := Sum([]byte("test"), SHA2_256, 33)
+	if err != ErrLenTooLarge {
+		t.Fatal("bad error", err)
+	}
+}