diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9f57bfde..699dbfdc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,6 +26,10 @@ All notable changes to this project will be documented in this file.
 
 ## Unreleased
 
+### Fixed
+
+- fix inline definition of `logging_level_to_string`
+
 ## [0.9.0] - 2024-09-19
 
 ### Added
diff --git a/README.md b/README.md
index 88e2dd10..840d548f 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# HTOOL [![CI](https://github.com/htool-ddm/htool/actions/workflows/CI.yml/badge.svg?branch=develop)](https://github.com/htool-ddm/htool/actions/workflows/CI.yml) [![codecov](https://codecov.io/gh/htool-ddm/htool/branch/main/graph/badge.svg?token=1JJ40GPFA5)](https://codecov.io/gh/htool-ddm/htool)
+# HTOOL [![CI](https://github.com/htool-ddm/htool/actions/workflows/CI.yml/badge.svg)](https://github.com/htool-ddm/htool/actions/workflows/CI.yml) [![codecov](https://codecov.io/gh/htool-ddm/htool/branch/main/graph/badge.svg?token=1JJ40GPFA5)](https://codecov.io/gh/htool-ddm/htool)
 
 ## What is Htool?
 
diff --git a/include/htool/hmatrix/hmatrix_distributed_output.hpp b/include/htool/hmatrix/hmatrix_distributed_output.hpp
index f5e40ad3..6f671eb3 100644
--- a/include/htool/hmatrix/hmatrix_distributed_output.hpp
+++ b/include/htool/hmatrix/hmatrix_distributed_output.hpp
@@ -23,7 +23,7 @@
 
 namespace htool {
 
-bool is_positive_integer(const std::string &s) {
+inline bool is_positive_integer(const std::string &s) {
     return !s.empty() && std::find_if(s.begin(), s.end(), [](unsigned char c) { return !std::isdigit(c); }) == s.end();
 }
 
diff --git a/include/htool/misc/logger.hpp b/include/htool/misc/logger.hpp
index 8dd9f27c..b8c42789 100644
--- a/include/htool/misc/logger.hpp
+++ b/include/htool/misc/logger.hpp
@@ -17,7 +17,7 @@ enum class LogLevel : unsigned int {
     INFO     = 40,
 };
 
-std::string logging_level_to_string(LogLevel logging_level) {
+inline std::string logging_level_to_string(LogLevel logging_level) {
     switch (logging_level) {
     case LogLevel::CRITICAL:
         return "[Htool critical] ";