diff --git a/PSOA2X/src/main/antlr3/org/ruleml/psoa/psoa2x/psoa2prolog/PrologConverter.g b/PSOA2X/src/main/antlr3/org/ruleml/psoa/psoa2x/psoa2prolog/PrologConverter.g
index d3de95cb..2a7872df 100644
--- a/PSOA2X/src/main/antlr3/org/ruleml/psoa/psoa2x/psoa2prolog/PrologConverter.g
+++ b/PSOA2X/src/main/antlr3/org/ruleml/psoa/psoa2x/psoa2prolog/PrologConverter.g
@@ -253,6 +253,10 @@ psoa
                     else
                        replace(startIdx, 7, "sloterm");
                  }
+		 else
+		 {
+                    throw new TranslatorException("Slotted expressions are not supported");
+		 }
                  append(")");
               }
            |  // No slots or tuples  
@@ -311,4 +315,4 @@ constshort
     |   LITERAL { convertStringConst($LITERAL.text); }
     |   NUMBER  { append($NUMBER.text); }
     |   LOCAL   { convertLocalConst($LOCAL.text); }
-    ;
\ No newline at end of file
+    ;