diff --git a/CHANGELOG-rust.md b/CHANGELOG-rust.md
index 8af2dbd..3d68df1 100644
--- a/CHANGELOG-rust.md
+++ b/CHANGELOG-rust.md
@@ -5,6 +5,10 @@ This changelog tracks the Rust `svdtools` project. See
 
 ## [Unreleased]
 
+## [v0.4.2] 2025-01-25
+
+* Fix optional `fspec` in `set_field_modified_write_values`
+
 ## [v0.4.1] 2025-01-21
 
 * `html`: field `readAction` and `modifiedWriteValues` in `access`
@@ -205,7 +209,8 @@ Other changes:
 
 * Initial release with feature-parity with the Python project.
 
-[Unreleased]: https://github.com/rust-embedded/svdtools/compare/v0.4.1...HEAD
+[Unreleased]: https://github.com/rust-embedded/svdtools/compare/v0.4.2...HEAD
+[v0.4.2]: https://github.com/rust-embedded/svdtools/compare/v0.4.1...v0.4.2
 [v0.4.1]: https://github.com/rust-embedded/svdtools/compare/v0.4.0...v0.4.1
 [v0.4.0]: https://github.com/rust-embedded/svdtools/compare/v0.3.21...v0.4.0
 [v0.3.21]: https://github.com/rust-embedded/svdtools/compare/v0.3.20...v0.3.21
diff --git a/Cargo.toml b/Cargo.toml
index 918e623..c699e72 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "svdtools"
-version = "0.4.1"
+version = "0.4.2"
 repository = "https://github.com/rust-embedded/svdtools/"
 description = "Tool for modifying bugs in CMSIS SVD"
 authors = ["Andrey Zgarbul <zgarbul.andrey@gmail.com>", "MarcoIeni"]
diff --git a/src/patch/register.rs b/src/patch/register.rs
index 9b62b26..36e08b7 100644
--- a/src/patch/register.rs
+++ b/src/patch/register.rs
@@ -711,12 +711,14 @@ impl RegisterExt for Register {
     }
 
     fn set_field_read_action(&mut self, fspec: &str, action: ReadAction) {
+        let (fspec, _) = fspec.spec();
         for ftag in self.iter_fields(fspec) {
             ftag.read_action = Some(action);
         }
     }
 
     fn set_field_modified_write_values(&mut self, fspec: &str, mwv: ModifiedWriteValues) {
+        let (fspec, _) = fspec.spec();
         for ftag in self.iter_fields(fspec) {
             ftag.modified_write_values = if mwv == ModifiedWriteValues::Modify {
                 None