diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 844e2f8..b31abf8 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -671,11 +671,21 @@ if(TARGET tr31-tool AND BUILD_TESTING)
 			PASS_REGULAR_EXPRESSION "D1840S0ES00N0400CT000405D6020002F0MIICLjCCAdSgAwIBAgIIGDrdWBxuNpAwCgYIKoZIzj0EAwIwMTEXMBUGA1UECgwOQWxwaGEgTWVyY2hhbnQxFjAUBgNVBAMMDVNhbXBsZSBFQ0MgQ0EwHhcNMjAwODE1MDIxMDEwWhcNMjEwODE1MDIxMDEwWjBPMRcwFQYDVQQKDA5BbHBoYSBNZXJjaGFudDEfMB0GA1UECwwWVExTIENsaWVudCBDZXJ0aWZpY2F0ZTETMBEGA1UEAwwKMTIzNDU2Nzg5MDBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABEI/SLrH6fITA9y6Y3BneuoT/5\\+EHSepZxCYeSstGll2sVvmSDZWWSbN6lh5Fb/zagrDjjQ/gZtWIOTf2wL1vSGjgbcwgbQwCQYDVR0TBAIwADAOBgNVHQ8BAf8EBAMCB4AwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0OBBYEFHuvP526vFMywEoVoXZ5aXNfhnfeMB8GA1UdIwQYMBaAFI\\+ZFhOWF\\+oMtcfYwg15vH5WmWccMEIGA1UdHwQ7MDkwN6A1oDOGMWh0dHA6Ly9jcmwuYWxwaGEtbWVyY2hhbnQuZXhhbXBsZS9TYW1wbGVFQ0NDQS5jcmwwCgYIKoZIzj0EAwIDSAAwRQIhAPuWWvCTmOdvQzUjCUmTX7H4sX4Ebpw\\+CI\\+aOQLu1DqwAiA0eR4FdMtvXV4P6\\+WMz5B10oea5xtLTfSgoBDoTkvKYQ==0002C4MIICDjCCAbOgAwIBAgIIfnOsCbsxHjwwCgYIKoZIzj0EAwIwNjEXMBUGA1UECgwOQWxwaGEgTWVyY2hhbnQxGzAZBgNVBAMMElNhbXBsZSBSb290IEVDQyBDQTAeFw0yMDA4MTUwMjEwMDlaFw0zMDA4MTMwMjEwMDlaMDExFzAVBgNVBAoMDkFscGhhIE1lcmNoYW50MRYwFAYDVQQDDA1TYW1wbGUgRUNDIENBMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEHCanM9n\\+Rji\\+3EROj\\+HlogmXMU1Fk1td7N3I/8rfFnre1GwWCUqXSePHxwQ9DRHCV3oht3OUU2kDfitfUIujA6OBrzCBrDASBgNVHRMBAf8ECDAGAQH/AgEAMA4GA1UdDwEB/wQEAwIBhjAdBgNVHQ4EFgQUj5kWE5YX6gy1x9jCDXm8flaZZxwwHwYDVR0jBBgwFoAUvElIifFlt6oeUaopV9Y0lJtyPVQwRgYDVR0fBD8wPTA7oDmgN4Y1aHR0cDovL2NybC5hbHBoYS1tZXJjaGFudC5leGFtcGxlL1NhbXBsZVJvb3RFQ0NDQS5jcmwwCgYIKoZIzj0EAwIDSQAwRgIhALT8\\+DG\\+\\+\\+KuqqUGyBQ4YG4s34fqbujclxZTHxYWVVSNAiEAn3v5Xmct7fkLpkjGexiHsy6D90r0K2LlUqpN/069y5s=010004asdfKP10012331550BC9TS1320200818004100ZPB07"
 	)
 
+	# test export header containing optional blocks and padding, but not adding additional optional blocks
 	add_test(NAME tr31_tool_test40
 		COMMAND tr31-tool --kbpk 88E1AB2A2E3DD38C1FA039A536500CC8A87AB9D62DC92C01058FA79F44657DE6 --export 3F419E1CB7079442AA37474C2EFBF8B8 --export-header D1234M7HG00N0200HM0621PB0A5V5E8F
 	)
 	set_tests_properties(tr31_tool_test40
 		PROPERTIES
-			PASS_REGULAR_EXPRESSION "^D0128M7HG00N0200HM0621PB0A5V5E8F"
+			PASS_REGULAR_EXPRESSION "^D0128M7HG00N0200HM0621PB0A5V5E8F" # note that optional block padding content is intentionally preserved
+	)
+
+	# test export header containing optional blocks and padding and also adding additional optional blocks
+	add_test(NAME tr31_tool_test41
+		COMMAND tr31-tool --kbpk 88E1AB2A2E3DD38C1FA039A536500CC8A87AB9D62DC92C01058FA79F44657DE6 --export 3F419E1CB7079442AA37474C2EFBF8B8 --export-header D1234M7HG00N0200HM0621PB0A5V5E8F --export-opt-block-LB "MyKey"
+	)
+	set_tests_properties(tr31_tool_test41
+		PROPERTIES
+			PASS_REGULAR_EXPRESSION "^D0144M7HG00N0300HM0621LB09MyKeyPB11"
 	)
 endif()
diff --git a/src/tr31.c b/src/tr31.c
index 9f7580b..4ebde5d 100644
--- a/src/tr31.c
+++ b/src/tr31.c
@@ -679,18 +679,44 @@ static struct tr31_opt_ctx_t* tr31_opt_block_alloc(
 )
 {
 	struct tr31_opt_ctx_t* opt_ctx;
+	bool opt_blk_pb_found = false;
 
 	if (!ctx) {
 		return NULL;
 	}
 
 	// repeated optional block IDs are not allowed
+	// and optional block PB must always be last
 	// see ANSI X9.143:2021, 6.3.6
 	for (size_t i = 0; i < ctx->opt_blocks_count; ++i) {
 		if (ctx->opt_blocks[i].id == id) {
 			// existing optional block found
 			return NULL;
 		}
+
+		if (ctx->opt_blocks[i].id == TR31_OPT_BLOCK_PB) {
+			// optional block PB found
+			opt_blk_pb_found = true;
+		}
+	}
+
+	// if optional block PB already exists, remove all instances
+	// NOTE: it will be recreated by tr31_export()
+	// NOTE: if no new optional blocks are added, PB is intentionally preserved
+	if (opt_blk_pb_found) {
+		for (size_t i = 0; i < ctx->opt_blocks_count; ++i) {
+			if (ctx->opt_blocks[i].id == TR31_OPT_BLOCK_PB) {
+				free(ctx->opt_blocks[i].data);
+				ctx->opt_blocks[i].data = NULL;
+
+				ctx->opt_blocks_count -= 1;
+				if (i < ctx->opt_blocks_count) {
+					size_t remaining_count = ctx->opt_blocks_count - i;
+					size_t remaining_bytes = sizeof(*ctx->opt_blocks) * remaining_count;
+					memmove(&ctx->opt_blocks[i], &ctx->opt_blocks[i + 1], remaining_bytes);
+				}
+			}
+		}
 	}
 
 	// grow optional block array