diff --git a/sum.go b/sum.go
index 53c1805..c18deb1 100644
--- a/sum.go
+++ b/sum.go
@@ -2,6 +2,7 @@ package multihash
 
 import (
 	"errors"
+	"fmt"
 )
 
 // ErrSumNotSupported is returned when the Sum function code is not implemented
@@ -27,6 +28,7 @@ func Sum(data []byte, code uint64, length int) (Multihash, error) {
 	sum := hasher.Sum(nil)
 
 	// Deal with any truncation.
+	//  Unless it's an identity multihash.  Those have different rules.
 	if length < 0 {
 		length = hasher.Size()
 	}
@@ -34,6 +36,11 @@ func Sum(data []byte, code uint64, length int) (Multihash, error) {
 		return nil, ErrLenTooLarge
 	}
 	if length >= 0 {
+		if code == IDENTITY {
+			if length != len(sum) {
+				return nil, fmt.Errorf("the length of the identity hash (%d) must be equal to the length of the data (%d)", length, len(sum))
+			}
+		}
 		sum = sum[:length]
 	}
 
diff --git a/sum_test.go b/sum_test.go
index c2e9c1b..512231c 100644
--- a/sum_test.go
+++ b/sum_test.go
@@ -123,6 +123,33 @@ func BenchmarkBlake2B(b *testing.B) {
 	}
 }
 
+func TestSmallerLengthHashID(t *testing.T) {
+
+	data := []byte("Identity hash input data.")
+	dataLength := len(data)
+
+	// Normal case: `length == len(data)`.
+	_, err := multihash.Sum(data, multihash.ID, dataLength)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Unconstrained length (-1): also allowed.
+	_, err = multihash.Sum(data, multihash.ID, -1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Any other variation of those two scenarios should fail.
+	for l := (dataLength - 1); l >= 0; l-- {
+		_, err = multihash.Sum(data, multihash.ID, l)
+		if err == nil {
+			t.Fatal(fmt.Sprintf("identity hash of length %d smaller than data length %d didn't fail",
+				l, dataLength))
+		}
+	}
+}
+
 func TestTooLargeLength(t *testing.T) {
 	_, err := multihash.Sum([]byte("test"), multihash.SHA2_256, 33)
 	if err != multihash.ErrLenTooLarge {