Index: tcc4tcl.tcl
==================================================================
--- tcc4tcl.tcl
+++ tcc4tcl.tcl
@@ -310,10 +310,19 @@
 	#   string    (TCL_DYNAMIC char*)
 	#   dstring   (TCL_DYNAMIC char*)
 	#   vstring   (TCL_VOLATILE char*)
 	#   default   (Tcl_Obj*)
 	#   wide
+	switch -- $rtype {
+		void - ok - int - long - float - double - wide {}
+		default {
+			append cbody "  if (rv == NULL) {\n"
+			append cbody "    return(TCL_ERROR);\n"
+			append cbody "  }\n"
+		}
+	}
+
 	switch -- $rtype {
 		void           { }
 		ok             { append cbody "  return rv;" "\n" }
 		int            { append cbody "  Tcl_SetIntObj(Tcl_GetObjResult(ip), rv);" "\n" }
 		long           { append cbody "  Tcl_SetLongObj(Tcl_GetObjResult(ip), rv);" "\n" }

Index: test
==================================================================
--- test
+++ test
@@ -28,18 +28,36 @@
 		return(TCL_ERROR);
 	};
 	return(TCL_OK);
 }
 
+# Return error on NULL
+tcc4tcl::cproc test4 {int v} char* {
+	if (v == 1) {
+		return("ok");
+	}
+
+	return(NULL);
+}
 
 puts [test 1]
 puts [test1 1]
 puts [test3 1]
 puts [::bob::test1 1]
 puts [add [test 1] 1]
+puts [test4 1]
 
 catch {
 	puts [mkdir "/"]
 } err
 if {$err != "failed"} {
 	error "\[mkdir\] did not return the expected error"
 }
+
+catch {
+	set v 0
+	puts [test4 0]
+	set v 1
+} err
+if {$err != "" || $v == 1} {
+	error "\[test4\] did not return the expected error"
+}