diff --git a/src/api/capi.h b/src/api/capi.h
index 0b5807fc3e..e053a9e83c 100644
--- a/src/api/capi.h
+++ b/src/api/capi.h
@@ -326,7 +326,7 @@ TESS_API TessOcrEngineMode
 
 TESS_API void  TESS_CALL TessBaseAPIInitTruthCallback(TessBaseAPI* handle, TessTruthCallback* cb);
 
-TESS_API void  TESS_CALL TessBaseGetBlockTextOrientations(TessBaseAPI* handle, int** block_orientation, BOOL** vertical_writing);
+TESS_API void  TESS_CALL TessBaseGetBlockTextOrientations(TessBaseAPI* handle, int** block_orientation, bool** vertical_writing);
 
 #endif