diff --git a/CHANGELOG.md b/CHANGELOG.md
index f47e420..c4010c5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `cmd/mp4ff-encrypt` did not parse command line
 - `SeigSampleGroupEntry` calculated skipBytes incorrectly
 - `cmd/mp4ff-pslister` did not parse annex B HEVC correctly
+- error when decrypting and re-encrypting a segement (issue #378)
 
 ### Removed
 
diff --git a/mp4/crypto.go b/mp4/crypto.go
index 7a069c8..2e74d4e 100644
--- a/mp4/crypto.go
+++ b/mp4/crypto.go
@@ -596,6 +596,9 @@ func DecryptFragment(frag *Fragment, di DecryptInfo, key []byte) error {
 			trun.DataOffset -= int32(nrBytesRemoved)
 		}
 	}
+	if frag.Mdat.StartPos > frag.Moof.StartPos {
+		frag.Mdat.StartPos -= nrBytesRemoved
+	}
 
 	return nil
 }
diff --git a/mp4/crypto_test.go b/mp4/crypto_test.go
index 4dfdf92..b74433f 100644
--- a/mp4/crypto_test.go
+++ b/mp4/crypto_test.go
@@ -213,6 +213,22 @@ func TestEncryptDecrypt(t *testing.T) {
 			if !bytes.Equal(rawSeg, decSegBuf.Bytes()) {
 				t.Errorf("segment not equal after encryption+decryption")
 			}
+
+			// Make a new encryption to check that the decrypted segment is OK
+			// for re-encryption (Issue #378).
+
+			pd2, err := InitProtect(encInit.Init, key, iv, c.scheme, kidUUID, nil)
+			if err != nil {
+				t.Error(err)
+			}
+			for _, s := range decode.Segments {
+				for _, f := range s.Fragments {
+					err := EncryptFragment(f, key, iv, pd2)
+					if err != nil {
+						t.Errorf("Error re-encrypting fragment: %v\n", err)
+					}
+				}
+			}
 		})
 	}
 }
@@ -238,3 +254,7 @@ func TestDecryptInit(t *testing.T) {
 		}
 	}
 }
+
+func TestDecryptEncrypt(t *testing.T) {
+
+}