diff -up libdap-3.18.1/unit-tests/D4FilterClauseTest.cc.getopt libdap-3.18.1/unit-tests/D4FilterClauseTest.cc
--- libdap-3.18.1/unit-tests/D4FilterClauseTest.cc.getopt	2016-05-05 17:43:15.000000000 -0600
+++ libdap-3.18.1/unit-tests/D4FilterClauseTest.cc	2016-08-12 14:00:24.398185600 -0600
@@ -566,7 +566,7 @@ int main(int argc, char*argv[]) {
     runner.addTest(CppUnit::TestFactoryRegistry::getRegistry().makeTest());
 
     GetOpt getopt(argc, argv, "d");
-    char option_char;
+    int option_char;
 
     while ((option_char = getopt()) != EOF)
         switch (option_char) {