diff --git a/public/crtlib.c b/public/crtlib.c
index 9c2b55e1..796f2ca2 100644
--- a/public/crtlib.c
+++ b/public/crtlib.c
@@ -929,12 +929,12 @@ void COM_TrimSpace( char *dst, const char *src, size_t size )
 		return;
 
 	// remove spaces from the start
-	for( ; *src && isspace( *src ); src++ );
+	for( ; *src && isspace((byte)*src ); src++ );
 
 	int len = Q_strlen( src );
 
 	// remove spaces from the end
-	for( ; len > 0 && isspace( src[len - 1] ); len-- );
+	for( ; len > 0 && isspace((byte)src[len - 1] ); len-- );
 
 	if( len > 0 )
 	{
diff --git a/public/crtlib.h b/public/crtlib.h
index f9a8b893..ae87aced 100644
--- a/public/crtlib.h
+++ b/public/crtlib.h
@@ -170,9 +170,15 @@ static inline qboolean Q_istype( const char *str, int (*istype)( int c ))
 {
 	if( likely( str && *str ))
 	{
-		while( istype( *str )) str++;
-		if( !*str ) return true;
+		// a1ba: explicitly cast char to unsigned char to not trigger UB
+		// in ctype functions
+		while( istype((byte)*str ))
+			str++;
+
+		if( !*str )
+			return true;
 	}
+
 	return false;
 }