diff --git a/src/vmime/charsetConverter_icu.cpp b/src/vmime/charsetConverter_icu.cpp
index cc74be98..22dd0df4 100644
--- a/src/vmime/charsetConverter_icu.cpp
+++ b/src/vmime/charsetConverter_icu.cpp
@@ -453,7 +453,7 @@ void charsetFilteredOutputStream_icu::writeImpl(
 
 			ucnv_fromUnicode(
 				m_to, &cpTarget, cpTargetLimit,
-				&cpSource, cpSourceLimit, NULL, /* flush */ FALSE, &fromErr
+				&cpSource, cpSourceLimit, NULL, /* flush */ UBOOL(0), &fromErr
 			);
 
 			if (fromErr != U_BUFFER_OVERFLOW_ERROR && U_FAILURE(fromErr)) {