diff --git a/iocBoot/iocimagedemo/st.cmd b/iocBoot/iocimagedemo/st.cmd
index 3fbb80d..325bf04 100755
--- a/iocBoot/iocimagedemo/st.cmd
+++ b/iocBoot/iocimagedemo/st.cmd
@@ -4,4 +4,6 @@ dbLoadRecords("image.db","N=TST:image1")
 dbLoadRecords("table.db","N=TST:table1")
 dbLoadRecords("ntenum.db","P=TST:enum1")
 
+dbLoadGroup("image.json")
+
 iocInit()
diff --git a/pdbApp/pdb.cpp b/pdbApp/pdb.cpp
index 8885090..3f747dd 100644
--- a/pdbApp/pdb.cpp
+++ b/pdbApp/pdb.cpp
@@ -239,7 +239,7 @@ struct PDBProcessor
 
 #ifdef USE_MULTILOCK
             try {
-                GroupConfig::parse(json, "", conf);
+                GroupConfig::parse(json, NULL, conf);
                 if(!conf.warning.empty())
                     fprintf(stderr, "warning(s) from dbGroup file \"%s\"\n%s", it->c_str(), conf.warning.c_str());
             }catch(std::exception& e){
diff --git a/pdbApp/pvif.cpp b/pdbApp/pvif.cpp
index 76def06..6f26d9c 100644
--- a/pdbApp/pvif.cpp
+++ b/pdbApp/pvif.cpp
@@ -40,19 +40,23 @@ namespace pva = epics::pvAccess;
 
 DBCH::DBCH(dbChannel *ch) :chan(ch)
 {
+    if(!chan)
+        throw std::invalid_argument("NULL channel");
     prepare();
 }
 
 DBCH::DBCH(const std::string& name)
     :chan(dbChannelCreate(name.c_str()))
 {
+    if(!chan)
+        throw std::invalid_argument(SB()<<"invalid channel: "<<name);
     prepare();
 }
 
 void DBCH::prepare()
 {
     if(!chan)
-        throw std::invalid_argument("NULL channel");
+        throw std::invalid_argument(SB()<<"NULL channel");
     if(dbChannelOpen(chan)) {
         dbChannelDelete(chan);
         throw std::invalid_argument(SB()<<"Failed to open channel "<<dbChannelName(chan));