diff --git a/src/support/lstrings.cpp b/src/support/lstrings.cpp
index 6c48a90ee4..8568b2f7e0 100644
--- a/src/support/lstrings.cpp
+++ b/src/support/lstrings.cpp
@@ -248,7 +248,7 @@ bool isAscii(docstring const & str)
 {
 	int const len = str.length();
 	for (int i = 0; i < len; ++i)
-		if (static_cast<unsigned char>(str[i]) >= 0x80)
+		if (str[i] >= 0x80)
 			return false;
 	return true;
 }