diff --git a/src/training/unicharset/unicharset_training_utils.cpp b/src/training/unicharset/unicharset_training_utils.cpp
index 63075345cf..853bee8e8e 100644
--- a/src/training/unicharset/unicharset_training_utils.cpp
+++ b/src/training/unicharset/unicharset_training_utils.cpp
@@ -38,7 +38,7 @@ namespace tesseract {
 // Helper sets the character attribute properties and sets up the script table.
 // Does not set tops and bottoms.
 void SetupBasicProperties(bool report_errors, bool decompose, UNICHARSET *unicharset) {
-  for (int unichar_id = 0; unichar_id < unicharset->size(); ++unichar_id) {
+  for (size_t unichar_id = 0; unichar_id < unicharset->size(); ++unichar_id) {
     // Convert any custom ligatures.
     const char *unichar_str = unicharset->id_to_unichar(unichar_id);
     for (int i = 0; UNICHARSET::kCustomLigatures[i][0] != nullptr; ++i) {