diff --git a/OpenVPN Adapter/OpenVPNCertificate.m b/OpenVPN Adapter/OpenVPNCertificate.m
index ca23293..d7412b2 100644
--- a/OpenVPN Adapter/OpenVPNCertificate.m	
+++ b/OpenVPN Adapter/OpenVPNCertificate.m	
@@ -102,11 +102,13 @@
 
 - (NSData *)derData:(out NSError **)error {
     if (self.crt->raw.p == NULL || self.crt->raw.len == 0) {
-        NSString *reason = [NSError reasonFromResult:MBEDTLS_ERR_X509_BAD_INPUT_DATA];
-        *error = [NSError errorWithDomain:OpenVPNIdentityErrorDomain code:MBEDTLS_ERR_X509_BAD_INPUT_DATA userInfo:@{
-            NSLocalizedDescriptionKey: @"Failed to write DER data.",
-            NSLocalizedFailureReasonErrorKey:reason
-        }];
+        if (error) {
+            NSString *reason = [NSError reasonFromResult:MBEDTLS_ERR_X509_BAD_INPUT_DATA];
+            *error = [NSError errorWithDomain:OpenVPNIdentityErrorDomain code:MBEDTLS_ERR_X509_BAD_INPUT_DATA userInfo:@{
+                NSLocalizedDescriptionKey: @"Failed to write DER data.",
+                NSLocalizedFailureReasonErrorKey:reason
+            }];
+        }
         
         return nil;
     }