diff --git a/src/factory/printer.cpp b/src/factory/printer.cpp
index 928fc4f..55fb01c 100644
--- a/src/factory/printer.cpp
+++ b/src/factory/printer.cpp
@@ -497,7 +497,7 @@ std::ostream& operator<<(std::ostream& strm, const escape& Q)
         case '\'': next = '\''; break;
         case '\"': next = '\"'; if(Q.S==escape::CSV) quote = '"'; break;
         default:
-            if(!isprint(C)) {
+            if(!isprint((unsigned char)C)) {
                 // print three charator escape
                 strm<<"\\x"<<hexdigit(C>>4)<<hexdigit(C);
             } else {
@@ -534,7 +534,7 @@ std::ostream& operator<<(std::ostream& strm, const maybeQuote& q)
             esc = true;
             break;
         default:
-            if(!isprint(q.s[i])) {
+            if(!isprint((unsigned char)q.s[i])) {
                 esc = true;
             }
             break;
diff --git a/testApp/misc/testprinter.cpp b/testApp/misc/testprinter.cpp
index 417dd5e..767b917 100644
--- a/testApp/misc/testprinter.cpp
+++ b/testApp/misc/testprinter.cpp
@@ -247,7 +247,7 @@ void showNTTable()
     iarr.push_back(42); // will not be shown
     input->getSubFieldT<pvd::PVIntArray>("value.colA")->replace(pvd::freeze(iarr));
 
-    sarr.push_back("one\x7f");
+    sarr.push_back("one\x7f\x80");
     sarr.push_back("two words");
     sarr.push_back("A '\"'");
     input->getSubFieldT<pvd::PVStringArray>("value.colB")->replace(pvd::freeze(sarr));
@@ -255,7 +255,7 @@ void showNTTable()
 
     testDiff("<undefined>                \n"
              "labelA   \"label B\"\n"
-             "     1     one\\x7F\n"
+             "     1 one\\x7F\\x80\n"
              "     2 \"two words\"\n"
              "     3  \"A \\'\"\"\\'\"\n"
              , print(input->stream()),