diff --git a/ZNCString.cpp b/ZNCString.cpp
index 8c511cbc..d8e0e9fa 100644
--- a/ZNCString.cpp
+++ b/ZNCString.cpp
@@ -1098,12 +1098,15 @@ static const char hexdigits[] = "0123456789abcdef";
 CString& MCString::Encode(CString& sValue) const {
 	CString sTmp;
 	for (CString::iterator it = sValue.begin(); it != sValue.end(); ++it) {
-		if (isalnum(*it)) {
-			sTmp += *it;
+		// isalnum() needs unsigned char as argument and this code
+		// assumes unsigned, too.
+		unsigned char c = *it;
+		if (isalnum(c)) {
+			sTmp += c;
 		} else {
 			sTmp += "%";
-			sTmp += hexdigits[*it >> 4];
-			sTmp += hexdigits[*it & 0xf];
+			sTmp += hexdigits[c >> 4];
+			sTmp += hexdigits[c & 0xf];
 			sTmp += ";";
 		}
 	}