diff --git a/src/idl_parser.cpp b/src/idl_parser.cpp
index dc8f58132..e89a39151 100644
--- a/src/idl_parser.cpp
+++ b/src/idl_parser.cpp
@@ -280,7 +280,7 @@ CheckedError Parser::Next() {
         int unicode_high_surrogate = -1;
 
         while (*cursor_ != c) {
-          if (*cursor_ < ' ' && *cursor_ >= 0)
+          if (*cursor_ < ' ' && static_cast<signed char>(*cursor_) >= 0)
             return Error("illegal character in string constant");
           if (*cursor_ == '\\') {
             cursor_++;