diff --git a/include/dwarfs/logger.h b/include/dwarfs/logger.h
index 2970736d..52d8fae1 100644
--- a/include/dwarfs/logger.h
+++ b/include/dwarfs/logger.h
@@ -21,12 +21,11 @@
 
 #pragma once
 
-#include <array>
 #include <atomic>
 #include <chrono>
 #include <cstddef>
 #include <ctime>
-#include <iostream>
+#include <iosfwd>
 #include <memory>
 #include <mutex>
 #include <optional>
@@ -59,10 +58,7 @@ class logger {
     TRACE
   };
 
-  static char level_char(level_type level) {
-    static std::array<char, 7> lchars = {{'F', 'E', 'W', 'I', 'V', 'D', 'T'}};
-    return lchars.at(level);
-  }
+  static char level_char(level_type level);
 
   virtual ~logger() = default;
 
diff --git a/include/dwarfs/writer/internal/detail/multi_queue_block_merger_impl.h b/include/dwarfs/writer/internal/detail/multi_queue_block_merger_impl.h
index 64350495..160ae34c 100644
--- a/include/dwarfs/writer/internal/detail/multi_queue_block_merger_impl.h
+++ b/include/dwarfs/writer/internal/detail/multi_queue_block_merger_impl.h
@@ -25,6 +25,7 @@
 #include <cassert>
 #include <condition_variable>
 #include <deque>
+#include <iostream>
 #include <mutex>
 #include <optional>
 #include <stdexcept>
diff --git a/src/logger.cpp b/src/logger.cpp
index 4aa60d82..986d55c3 100644
--- a/src/logger.cpp
+++ b/src/logger.cpp
@@ -25,6 +25,7 @@
 #include <stdexcept>
 
 #include <folly/Conv.h>
+#include <folly/lang/Assume.h>
 #include <folly/small_vector.h>
 
 #include <dwarfs/config.h>
@@ -61,6 +62,26 @@ constexpr std::array<std::pair<std::string_view, logger::level_type>, 6>
 
 }
 
+char logger::level_char(level_type level) {
+  switch (level) {
+  case FATAL:
+    return 'F';
+  case ERROR:
+    return 'E';
+  case WARN:
+    return 'W';
+  case INFO:
+    return 'I';
+  case VERBOSE:
+    return 'V';
+  case DEBUG:
+    return 'D';
+  case TRACE:
+    return 'T';
+  }
+  folly::assume_unreachable();
+}
+
 std::ostream& operator<<(std::ostream& os, logger::level_type const& optval) {
   return os << logger::level_name(optval);
 }
diff --git a/src/writer/categorizer.cpp b/src/writer/categorizer.cpp
index 813555e1..e50c4e80 100644
--- a/src/writer/categorizer.cpp
+++ b/src/writer/categorizer.cpp
@@ -20,6 +20,7 @@
  */
 
 #include <cassert>
+#include <iostream>
 #include <unordered_map>
 
 #include <boost/program_options.hpp>
diff --git a/test/test_logger.h b/test/test_logger.h
index 06263665..7e0e4fb8 100644
--- a/test/test_logger.h
+++ b/test/test_logger.h
@@ -22,6 +22,7 @@
 #pragma once
 
 #include <cstdlib>
+#include <iostream>
 #include <mutex>
 #include <optional>
 #include <string>