diff --git a/src/UglyToad.PdfPig.Tests/Integration/EncryptedDocumentTests.cs b/src/UglyToad.PdfPig.Tests/Integration/EncryptedDocumentTests.cs
index 58cd342bb..04eccdd8c 100644
--- a/src/UglyToad.PdfPig.Tests/Integration/EncryptedDocumentTests.cs
+++ b/src/UglyToad.PdfPig.Tests/Integration/EncryptedDocumentTests.cs
@@ -63,6 +63,15 @@ public void CanReadDocumentWithEmptyStringEncryptedWithAESEncryptionAndOnlyIV()
             }
         }
 
+        [Fact]
+        public void CanReadDocumentWithNoKeyLengthAndRevision4()
+        {
+            using (var document = PdfDocument.Open(IntegrationHelpers.GetSpecificTestDocumentPath("r4_aesv2_no_length")))
+            {
+                Assert.Empty(document.Information.Producer);
+            }
+        }
+
         private static string GetPath() => IntegrationHelpers.GetSpecificTestDocumentPath(FileName);
     }
 }
\ No newline at end of file
diff --git a/src/UglyToad.PdfPig.Tests/Integration/SpecificTestDocuments/r4_aesv2_no_length.pdf b/src/UglyToad.PdfPig.Tests/Integration/SpecificTestDocuments/r4_aesv2_no_length.pdf
new file mode 100644
index 000000000..357f7fa72
Binary files /dev/null and b/src/UglyToad.PdfPig.Tests/Integration/SpecificTestDocuments/r4_aesv2_no_length.pdf differ
diff --git a/src/UglyToad.PdfPig/Encryption/EncryptionHandler.cs b/src/UglyToad.PdfPig/Encryption/EncryptionHandler.cs
index 1a6fa67be..91ce19498 100644
--- a/src/UglyToad.PdfPig/Encryption/EncryptionHandler.cs
+++ b/src/UglyToad.PdfPig/Encryption/EncryptionHandler.cs
@@ -82,8 +82,8 @@ public EncryptionHandler(EncryptionDictionary encryptionDictionary, TrailerDicti
 
                 cryptHandler = cryptHandlerLocal;
 
-                useAes = cryptHandlerLocal?.StreamDictionary?.Name == CryptDictionary.Method.AesV2
-                    || cryptHandlerLocal?.StreamDictionary?.Name == CryptDictionary.Method.AesV3;
+                useAes = cryptHandlerLocal.StreamDictionary.Name == CryptDictionary.Method.AesV2
+                    || cryptHandlerLocal.StreamDictionary.Name == CryptDictionary.Method.AesV3;
             }
 
             var charset = OtherEncodings.Iso88591;
@@ -96,7 +96,18 @@ public EncryptionHandler(EncryptionDictionary encryptionDictionary, TrailerDicti
 
             var length = encryptionDictionary.EncryptionAlgorithmCode == EncryptionAlgorithmCode.Rc4OrAes40BitKey
                 ? 5
-                : encryptionDictionary.KeyLength.GetValueOrDefault() / 8;
+                : encryptionDictionary.KeyLength switch
+                {
+                    // this is as per the PDF specification, (PDF_ISO_32000-2, 7.6.2 Application of encryption, table 20, indicating Length default value is 40)
+                    null when encryptionDictionary.EncryptionAlgorithmCode == EncryptionAlgorithmCode.Rc4OrAesGreaterThan40BitKey => 40,
+                    null when encryptionDictionary.EncryptionAlgorithmCode == EncryptionAlgorithmCode.UnpublishedAlgorithm40To128BitKey => 40,
+                    // this is based on a specific use case encountered and tested, in link with the pdf specification (PDF_ISO_32000-2, 7.6.5.3 Public-key encryption algorithms, table 25, CFM comments)
+                    null when cryptHandler?.StreamDictionary.Name == CryptDictionary.Method.AesV2 => 128,
+                    // this is speculation, in link with the pdf spec, in link with the pdf specification (PDF_ISO_32000-2, 7.6.5.3 Public-key encryption algorithms, table 25, CFM comments)
+                    null when cryptHandler?.StreamDictionary.Name == CryptDictionary.Method.AesV3 => 256,
+                    // other use cases
+                    _ => encryptionDictionary.KeyLength.GetValueOrDefault()
+                } / 8;
 
             var foundPassword = false;